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Mechanisms of groundwater recharge and pesticide penetration to a chalk aquifer in southern England
Journal of Hydrology 275 (2003) 122–137 www.elsevier.com/locate/jhydrol
Mechanisms of groundwater recharge and pesticide penetration to a chalk aquifer in southern England Atul H. Haria*, Martin G. Hodnett, Andrew C. Johnson Centre for Ecology and Hydrology (formerly Institute of Hydrology), Maclean Building, Crowmarsh Gifford, Wallingford, Oxfordshire OX10 8BB, UK Received 25 November 2002; accepted 7 January 2003
Abstract In order to assess the potential for enhanced agrochemical contamination of shallow groundwaters, a field study was established on the Upper Chalk in Hampshire, UK. Two instrumented sites, 380 m apart, were established on a hillslope; one on the interfluve with a deep water table (,18 m depth), the other on the dry valley bottom where the groundwater was close to the surface (, 4 m depth). Hourly measurements of water potentials in the unsaturated zone to 3.0 m depth identified very different groundwater recharge processes between the two sites in response to the same storm event. On the interfluve site with the deep water table only matrix flow through the chalk unsaturated zone at 3 m depth was identified. In contrast, at the dry valley bottom with a shallow water table, both rapid preferential flow and matrix flow processes were observed at a 3 m depth. The correlation between groundwater depth measurements and unsaturated profile moisture content measurements demonstrated the importance of the capillary fringe in sustaining a higher moisture content in the unsaturated zone at the shallow groundwater site. The resulting reduced water storage capacity for vertical drainage fluxes meant that little water was required to wet the shallow profile before rapid preferential flow events, demonstrated by rapid water potential responses, occurred. However, where the groundwater was deeper, ‘intermediate’ storage sites located on chalk surfaces and at chalk ped/block ‘contact points’ remained empty and unsaturated water potential profiles showed that rainfall pulses were attenuated as these sites absorbed the downward water fluxes. Consequently, preferential events at these deep groundwater sites are rare. The importance of these ‘intermediate’ storage sites in controlling recharge processes is highlighted. The potential mass load of pesticide transported to the shallow groundwater in preferential events during 1996– 7 was determined using bromide tracer studies, water balance calculations and measured groundwater pesticide concentrations. An estimated 0.1% of the applied pesticide reached the shallow groundwater along preferential pathways in 1996– 7. Calculations under a worst-case scenario showed that this value did not increase beyond 0.2% of pesticide applied. q 2003 Elsevier Science B.V. All rights reserved. Keywords: Chalk; Groundwater recharge; Unsaturated zone; Hillslope hydrology; Capillary fringe; Isoproturon; Chlotoluron
1. Introduction
* Corresponding author. Fax: þ 44-(0)-1491-692424. E-mail address: [email protected] (A.H. Haria).
Chalk aquifers of south-east England contribute 55% of all groundwater use in the UK (Lloyd, 1993). Large tracts of the typically thin soils overlying these
0022-1694/03/$ - see front matter q 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-1694(03)00017-9
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unconfined aquifers are under arable land use and the water quality of this potable resource remains an important issue. The impact of intensive agriculture on underlying groundwater was first realised in the mid-1970s when observed nitrate concentrations exceeded European Union permissible limits (Foster, 1993). Concern in recent years has spread to pesticide contamination following the introduction of the 0.1 mg/l drinking water limit by the EU (Headworth, 1989; Anonymous, 1995, 1999; Johnson et al., 2001). Chalk has been described as a dual porosity system where water flow can occur both in the chalk matrix and through the fractures between the chalk blocks (Price et al., 1993). The Upper Chalk, a sub-division of UK chalk based on lithology (the others being Lower and Middle Chalk), has specific physical properties. The maximum pore diameter of the Upper Chalk matrix is about 1.0 mm, with an associated intergranular hydraulic conductivity of about 0.001 m/day (Price et al., 1976). The porosity and permeability of the Upper Chalk matrix is greater than that of the Middle and Lower Chalk. Understanding the mechanisms of groundwater recharge is critical to assessing pollutant transfer to the water table. If water and solutes move rapidly through fractures then dilution, attenuation and degradation will be minimised and the contaminant is likely to arrive at relatively high concentrations soon after application on the soil surface. However, if transport is through the matrix then the much slower travel time will allow for dispersion, dilution and potential degradation (Johnson et al., 1998) which will influence the final concentration that reaches the groundwater. In addition to fracture flow and matrix flow a third mechanism of thin film flow (i.e. water film flow along fracture surfaces of porous rocks where the fractures are largely unsaturated) needs to be considered. In fractured porous media at potentials close to saturation, Tokunaga and Wan (1997) showed how thin film flows could be significant in moving water at fluxes about three orders in magnitude greater than matrix pore water under unit gradient saturated flow. Recently, Tokunaga and Wan (2001) have identified ‘surface-zone flows’ as another fast flow system in the enhanced porosity of the rock skin at the fracture surface. Vertical fracture flow in chalk will only begin when the vertical drainage flux exceeds the saturated
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hydraulic conductivity of the chalk matrix (Wellings, 1984a; Cooper et al., 1990; Price et al., 1993). Wellings (1984a) observed fracture flow only once in a five year period in the unsaturated zone over a deep groundwater site on the Upper Chalk in Hampshire. Although fractures have the ability to conduct large volumes of water quickly to depth, fracture flow is rare and the predominant mode of recharge is through the fine pores of the chalk matrix. Wellings (1984a) showed that fracture flow was generated only when matric chalk water potentials exceed a threshold value of 2 5 kPa. In a study on the Upper Chalk of the Berkshire downs, Barraclough et al. (1994) showed tracer profiles (15NO3, 2H2O and Cl2) that were also inconsistent with significant vertical macrofissure flow, and were explained by flow through the water filled porosity of the chalk matrix. A number of field studies conducted on sites with a thick unsaturated zone over a deep water table have measured rates of water movement through the chalk matrix to be about 1 m/yr (Wellings and Bell, 1980; Wellings, 1984b; Barraclough et al., 1994). However, to date, little work on the hydrology of shallow chalk groundwater systems has been conducted. Gillham (1984) described the effect of the capillary fringe on shallow groundwater responses and the implications for contaminant transport. The work, however, refers to homogeneous systems and not a dual porosity system like chalk. Recent work on the Upper Chalk over a shallow water table has shown how these groundwaters may receive sudden peaks of high pesticide concentration shortly after heavy rainfall (Johnson et al., 2001). This would suggest that these shallow water table sites may represent ‘hot spots’ for rapid pesticide transport to the groundwater. This paper describes a field study that was designed to determine the differences, if any, in recharge mechanisms between deep and shallow groundwater sites and whether this may lead to a greater contamination risk where the groundwater is close to the soil surface. Using a combination of hydrological measurements and bromide tracer studies an assessment of groundwater recharge and pesticide transport was made. Water balance calculations combined with pesticide concentrations measured in groundwater following rainfall were used to estimate the pesticide load arriving rapidly to the shallow
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groundwaters. The hypothesis of a field having different recharge mechanisms in different areas and therefore different risk potentials for groundwater contamination was addressed.
2. Site description The study area WON (Johnson et al., 1998) was located on the Upper Chalk in Hampshire, about 24 km north of Winchester and 1 km from a small tributary of the River Test. The study was located on a hillslope in an arable field on the chalk outcrop and comprised of an upper interfluve site and, at the foot of the slope (4 –5% gradient), a lower dry valley floor site. The dry valley floor site, subsequently referred to as WON 4, had a water table that was approximately 4 m from the soil surface and the interfluve site, subsequently referred to as WON 7, had a water table approximately 18 m below ground level. The distance between sites WON 4 and WON 7 was about 380 m (Fig. 1). The top 0.8 m of the profile at WON 4 is a silty clay loam soil described as Andover series (Jarvis et al., 1984), with increasing amounts of chalk mixed in below this depth; the soil-free chalk begins between 1.5 and 2.0 m below the soil surface. Chalk gravels, created by cryoturbation and other weathering processes, are found to a 3.0 m depth; the whole weathered chalk profile extending to 5 m below the surface. Additionally, chalk with a putty type structure, likely to be weathered chalk sludge washed down from the higher slopes, was also present in some areas. Observations from a 3 m deep pit at WON 7 showed a soil profile (Andover Series; Jarvis et al., 1984) with an average thickness of 0.3 m overlying a weathered chalk horizon (0.3 –1.0 m) with isolated patches of chalk gravels. Fracturing was observed to decrease with depth. During the 1996– 7 agricultural crop season the field was sown with grass, however, 5 £ 5 m plots over the boreholes were sown with wheat to permit pesticide use. Further details on the site and cropping patterns can be found elsewhere (Johnson et al., 1998; 2001).
3. Methods 3.1. Hydrological monitoring 3.1.1. Site WON 4 Soil and chalk water contents were recorded weekly through the 1996– 7 season using a neutron probe (Bell, 1976) in two access tubes spaced 0.5 m apart to a depth of 6 m. Readings were taken at 0.1 m depth increments to 0.5 m and then at 0.25 m depth increments to 6 m. Soil and chalk water potentials were measured using Pressure Transducer Tensiometers (PTTs) installed vertically by auguring through access tubes to ensure a watertight seal, to depths of 0.1, 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0 m. The PTTs were installed in two rows 0.5 m apart with a similar spacing between each PTT within the rows. The PTT site was situated in the corner of the field approximately 15 m from the closest field boundary and 3 m upslope of a 150 mm diameter observation borehole, WON 4. Rainfall was measured using a tipping bucket gauge and data were logged hourly. In addition to on-site rainfall measurements, MORECS (Thompson et al., 1981) rainfall and evaporation data were used for water balance calculations providing information outside the instrumented field season; farming practice meant field instruments had to be removed for part of the year. The permanent borehole at WON 4 was drilled on 12th November 1991 to 8 m below ground level using a dry percussion drilling method (Dixon, 1989). Sufficient slotted PVC casing (78 mm i.d.) was installed to accommodate the seasonal water level fluctuations. The annulus adjacent to the slots was infilled with clean gravel, followed by a 0.5 m sealing layer of bentonite above, and cement grout to the surface. The depth of the water table below the ground surface was recorded hourly using a pressure transducer interfaced to a data logger. Further boreholes (e.g. WON 5) were drilled close to WON 4 to provide additional chemical data (Johnson et al., 2001). 3.1.2. Site WON 7 The borehole at WON 7 was drilled using the same dry percussion method on 8th September 1996 to a depth of 20.71 m. WON 7, 77 m above sea level and 382 m upslope of WON 4, represented the site with
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Fig. 1. Schematic representation of the study area showing the deep groundwater site WON 7 and the shallow groundwater site WON 4.
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the greatest depth of unsaturated zone to the water table (18 m approx.) (Fig. 1). Water potentials were also measured at this site using Pressure transducer tensiometers (PTTs) installed to the same depths and in the same manner as for site WON 4. Data were logged hourly. 3.2. Bromide tracer studies On 22nd February 1995, 8 l of 50,000 mg/l KBr tracer solution was applied manually using a watering can with a rose spout in a 24 m2 grid 1 m upslope from borehole WON 4. Chalk cores of about 0.25 m length were extracted (on 28th September 1996 and 22nd September 1997) using a dry percussion sampling method on the site of application to assess, over time, bromide transport through the chalk matrix. Pore water from the cores was extracted by centrifugation using the method described by Kinniburgh and Miles (1983) and analysed for Br2 by ion chromatography (Dionex model 2010i). The eluent used was 1.8 mM sodium carbonate and 1.7 mM sodium bicarbonate. The regenerate used was 25 mM sulphuric acid. Detection was by electrical conductivity. Pore water Br2 concentrations for the profile were obtained.
4. Results and discussion 4.1. Hillslope recharge processes PTT data describing recharge processes through the unsaturated chalk profile at sites WON 4 and WON 7 were compared for a single rainfall event in 1997 (Storm 1, Table 1 and Fig. 2). The comparison was restricted to one storm event because of practical difficulties experienced with PTTs at WON 7; potentials at this site were very low for most of the season, and often below the minimum potential that can be recorded by tensiometers (2 80 kPa). Only Table 1 Rainfall data for Storm 1 and Storm 2 Event
Date
Rainfall (mm)
Duration (hrs)
Avg. intensity (mm/hr)
Storm 1 Storm 2
23–27 Feb 97 3–4 Mar 97
38 14.5
22 19
1.7 0.8
after air was purged from the PTTs insitu, were reliable readings obtained for short periods. It was fortunate that the PTTs at WON 7 were purged (denoted by ‘P’ in Fig. 2(b)) immediately prior to Storm 1. Data in Fig. 2(b) prior to purging, ‘P’, were unreliable and should be ignored. The need for regular purging of tensiometers at the deep groundwater site (WON 7) is clear evidence of highly negative potentials and drier profile to a 3 m depth compared with the shallow groundwater site (WON 4) where the wetter profile meant that the PTTs worked efficiently for the season. Different responses in groundwater recharge, between WON 4 and WON 7 unsaturated chalk zones, were observed for Storm 1 (Fig. 2). Unsaturated hydraulic potentials (0.5 –3.0 m depth) at WON 4 increased in two phases on 13th February 1997 and 18th February 1997 in response to rainfall. The increases were simultaneous from 1.5 to 3.0 m depths, marked A and B in Fig. 2(c)), resulting from piston displacement of downward water fluxes through the uniform porosity of the chalk matrix. The low chalk water potentials (more obvious in the deeper profile) mean that flow is through the matrix porosity only. The rate of change of potential in the chalk was greatest nearest the soil surface indicating a wetting pulse moving down the profile (Fig. 2(c)). The overall increase in water potentials show the gradual wetting of the profile by small rainfall events prior to Storm 1. Under these antecedent conditions, the rainfall associated with Storm 1 was of sufficient volume and intensity to allow the water flux to be transmitted quickly through the profile. The sharp (short duration) peaks in hydraulic potential in response to Storm 1 (Fig 2(c)) indicate larger fluxes over a short interval, compared to the preceding period. The peaks showing highest potential indicate when the hydraulic conductivity was at its highest. Depending on the gradient, the greatest flux will then occur at almost the same time. Similar findings in the Middle Chalk have been observed by Hodnett and Bell (1990). The anomaly arrowed X in Fig. 2(c) shows an early rise in potential clearly visible at 3.0 m and less so at 2.5 m. These may be explained either by decreased barometric pressure as a frontal system associated with rainfall passed over, causing instantaneous hydrostatic pressure responses at the water table in the borehole, or by the weight of rainwater on
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Fig. 2. Time series hydraulic potential responses to Storm 1 showing the attenuated response at WON 7 compared to the fast response at WON 4.
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the whole aquifer system causing isostatic responses in the borehole. Similar water level fluctuations were cited by Headworth (1972), in a review of groundwater level fluctuations in the chalk of Hampshire. Assuming the highest water potentials represent the peak water flux through the profile, Fig. 2(b) shows a much greater delay and attenuation in transit of peak water fluxes at the deep groundwater site WON 7, in response to Storm 1, than the sharp peaks in hydraulic potential observed at WON 4 (Fig. 2(c)). Peak water fluxes reached a 3.0 m depth in just over a day at the shallow groundwater site WON 4, whilst at WON 7 the peak flux arrived seven days after peak rainfall (Fig. 3). The hydrological response at the shallow groundwater site WON 4 to Storm 1 is more clearly demonstrated in Fig. 4(b), which shows the changes in hydraulic potential with depth from 11th to 26th February 1997. Following rainfall, the potentials increase, moving towards the line denoting
Fig. 3. Graph showing the delay in peak drainage flux with depth, after peak rainfall, at WON 7 compared to WON 4.
gravitational potential (saturation) until they become parallel with it at an approximate matric potential of 2 2.4 kPa during peak flux at 3.0 m depth (26th February 1997). The increase in potential indicates an increase in water content and therefore hydraulic conductivity to a point where the increasing downward water fluxes can be transmitted through the profile. Under unit gradient conditions, where the hydraulic potential profile is parallel to the line denoting gravitational potential as at WON 4 (Fig. 4(b)), the increase in drainage flux is proportional to the increase in hydraulic conductivity. Previous work on conductivity– potential relationships (Wellings, 1984a; Hodnett and Bell, 1990; Cooper et al., 1990; Tokunaga and Wan, 1997) has shown that small increases in water potential close to saturation can greatly increase the hydraulic conductivity. In this case a matric potential of 2 2.4 kPa is well within the threshold figure of 2 5 kPa for the onset of preferential fracture flow in the Upper Chalk as determined by Wellings (1984a). Changes in hydraulic potential with depth at the deep groundwater site WON 7, between 26th February 1997 and 3rd March 1997 (in response to Storm 1), are shown in Fig. 4(a). Matric potentials at 3.0 m depth did not exceed 2 17 kPa (well below the 2 5 kPa threshold for the initiation of fracture flow) during peak drainage (3rd March 1997) at this depth, showing that water flow was through the chalk matrix only. However, in the upper profile matric potentials as high as 2 2 kPa were observed indicating preferential or thin-film flows occurring down to a 1.0 m depth which were subsequently attenuated deeper in the chalk (discussed later). This indicates a lag in the drainage flux as downward water fluxes are ‘held’ and slowly released. At the surface this is likely to be a result of hydrodynamic dispersion in the soil horizon, however, the lag is also evident well below the soil zone (e.g. 2 – 3 m depth). Conventional thinking cannot explain this phenomenon since at the low potentials in question the fractures would all be empty and the uniform matrix porosity would all be waterfilled. The results highlight the existence of potentially important ‘intermediate’ storage sites (between the fracture and matrix porosities) where downward water fluxes can be ‘held’ and consequently reduced. A very small attenuation in drainage flux is observed at the shallow groundwater site (WON 4) suggesting
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Fig. 4. Change in hydraulic potential with depth during peak drainage flux in response to Storm 1 at (a) WON 7 and (b) WON 4.
that the ‘intermediate’ storage sites are largely waterfilled, so reducing the storage capacity of the profile. By the time peak drainage fluxes reached a depth of 2 m and below at WON 7, chalk water potentials at 1.5 m and above were becoming more negative, indicating the upper profile had started to dry out (Fig. 4(a)). Hence, the rainfall event which caused preferential flow at 2– 3 m at the shallow groundwater site (WON 4) resulted in matrix flow only through the 2 –3 m chalk at the deep groundwater site (WON 7). This is despite a thicker soil layer at site WON 4,
compared with WON 7, which might be expected to absorb and hold more of the rainfall releasing it more slowly to the chalk below. This study shows that it is likely to be a reduction in the water storage capacity at WON 4, itself likely to be a result of the shallow groundwater, which is the most important factor in governing the occurrence of preferential flow events. The effect of Storm 2 (Table 1) for both sites had less of an impact on recharge (Fig. 2). A unit potential gradient had already been established at the WON 4, and Storm 2 caused very slight rises in potential at 1.5
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and 2.0 m depths with no effect observed below this depth. Similarly, very little response to Storm 2 was observed below 2.0 m depth at WON 7. This emphasises the importance of individual storm characteristics in which, regardless of the antecedent conditions, certain threshold criteria of rainfall intensity and volume still need to be met before significant preferential/thin film flows can occur. For both Storms 1 and 2, surface PTTs (0.1 m depth) showed conditions remained unsaturated indicating the absence of runoff at either site. There was also no discernable response of the groundwater level at WON 4 in response to Storm 1 or Storm 2. There was a steady rise in groundwater level from 5.7 m below ground level to 4.7 m below ground level over the period shown in Fig. 2 (9th February 1997– 9th March 1997). Groundwater levels over the 1996/7 season fluctuated from 4.5 m below ground level to greater than 6 m below ground level. 4.2. Shallow groundwater effects Neutron probe data from WON 4 in the 1996– 7 season provides evidence of the influence of changes in shallow groundwater level on storage in the unsaturated chalk profile above. Moisture content data for 3 layers in the unsaturated chalk above the water table (2 –3 m, 3 –4 m and 4– 5 m depth layers) show that the layer closest to the groundwater (4 –5 m depth) was consistently wetter than the 3– 4 m layer above, which in turn was wetter than the 2 –3 m layer highest up (Fig. 5; time series data not shown). Correlating the moisture content in the 3 chalk layers with groundwater depth showed the greatest influence of the groundwater, throughout the 1996– 7 season, on the unsaturated chalk profile closest to the water table (Fig. 5). The coefficient of determination (r 2 ) is greatest for the layer closest to the groundwater, decreasing towards the surface layers. Significance F testing of groundwater on unsaturated storage showed an increasing influence with depth: 2 –3 m layer (0.024), 3 – 4 m layer (, 0.01) and 4 – 5 m layer ( ! 0.01). These data suggest that in the 1996 – 7 season groundwater had a significant impact on unsaturated chalk water storage to a depth of 3– 4 m, and a lesser impact above that depth.
4.3. Groundwater recharge and pesticide penetration at WON 4 Information on mean pore water velocity of recharge through the chalk matrix at the shallow groundwater site WON 4, between September 1996 and September 1997, was derived from the differences in bromide peak travel between the profiles in Fig. 6. Recovery of bromide from profile 1 was 104%, however, the forward tailing suggests this value is likely to be greater because of incomplete recovery. Recovery from profile 2 was 63%. The variability in bromide application by hand combined with the effects of micro-topography and the small sampling volume (0.1 m (4 inch) diameter cores) indicate that little can be concluded from this information. Certainly, more replicate cores needed to be taken at any one sampling period. The distance moved by the peak for profile 1, calculated as 2.17 m (the mid-point of the flat plateau), to the peak for profile 2 (3.31 m) was 1.14 m. The recharge through the chalk matrix required to move bromide peak 1 to peak 2 (Table 2, Fig. 6) was calculated based on three effective water filled porosity values of 0.43 v/v (100%), 0.37 v/v (85%, after Besien et al., 2000) and 0.22 v/v (50%, after Barraclough et al., 1994). The latter represents the worst-case scenario, with the more realistic results based on an effective porosity of 0.37 v/v calculated from breakthrough experiments on cores taken from site WON 4 (Besien et al., 2000). The remaining rainfall was assumed to be partitioned between crop interception/evaporation, soil water storage changes and rapid preferential drainage. MORECS (Thompson et al., 1981) data indicate that there was only 88 mm of recharge (rainfall minus evaporation) between the times of the two cored profiles. With a water content of 0.37, this would explain a downward movement of bromide of only about 0.24 m, compared with the observed movement of 1.14 m. It is therefore concluded that the recharge must have been greater than calculated by MORECS, probably because some of the evaporative demand was satisfied by the shallow groundwater. Movement of shallow groundwater into the unsaturated profile above, along upward gradients during the summer months, was identified by PTT data (not shown).
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Fig. 5. Correlation between moisture content in three unsaturated chalk layers and groundwater depth at WON 4 showing the greatest effect of the shallow groundwater on the deepest stratum.
In order to estimate the potential pesticide mass likely to reach the groundwater along preferential pathways for the period between cored bromide profiles, water balance calculations were combined with data of pesticide concentrations found in the groundwater. The evaporation component of the water balance was assumed to be negligible because: (a) the groundwater inputs satisfied much of the evaporative demand in the summer, (b) preferential recharge was likely to occur over a short duration during the wet winter months when evaporation would be lowest. Although information on crop interception was limited, estimates of canopy interception for winter wheat range from 30 to 40% where the leaf area index (LAI) is greater than 1 (Butler and Huband, 1985). However, in the 1996 –7 season, the field had been sown with grass and the expected interception would be less. Water balance calculations where rainfall from June to August 1997 (when LAI is greater than 1) was reduced by 30% did not make
significant differences to the pesticide load reaching the groundwater. Therefore, to bias the estimated pollution risk by preferential flow events to a worst-case scenario, the interception was assumed to be negligible. For water balance purposes, changes in profile water storage were also assumed to be negligible over the year because of capillary rise from the shallow groundwater. Weekly neutron probe data also showed very small changes in the chalk profile water content during the experimental season. The volume of rainfall not travelling through the chalk matrix (identified by the bromide profiles) was, therefore, allocated to preferential recharge to provide a worst-case scenario (Table 2). The spiked concentrations of pesticides Isoproturon (IPU) and Chlorotoluron (CTU), recorded over 3 years in the groundwater close to the water table (borehole WON 5) at site WON 4 by automated daily sampling after rainfall (Johnson et al., 2001), were used as a measure of
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Fig. 6. Bromide concentration profiles in the chalk unsaturated zone at WON 4 in September 1996 and September 1997.
the pesticide concentration in the water moving in the fractures. Although there will be some degree of dilution, these samples represented the best available estimates of the mass balance of pesticide preferentially reaching the groundwater. Based on the values observed by Johnson et al. (2001), estimates of potential pesticide concentrations likely to reach the groundwater by preferential fracture flow during the 1996 – 7 season are presented in Table 2. The findings suggest that, with an effective porosity of 0.37 v/v (Besien et al., 2000) and averaged pesticide spike concentrations (Johnson et al., 2001), approximately 0.1% IPU or CTU applied, preferentially reached the groundwater in the 1996 –7 season. Calculations under a worst-case scenario show that no more than 0.2% of applied IPU and CTU reached the groundwater in the 1996 –97 crop season by preferential flow
mechanisms. These results provide a field measured estimate of rapid pesticide loading to a shallow unconfined chalk aquifer. 4.4. Conceptual model of groundwater recharge processes Although chalk is considered to be a dual porosity system, this study has identified the effect of a third ‘intermediate’ porosity or storage site as the main controlling factor in recharge processes to shallow groundwater systems. This intermediate porosity can be described as that which is greater in size than the Upper Chalk matrix porosity and smaller than the fractures that become water-filled at potentials greater than 2 5 kPa matric suction. Note that the term ‘intermediate porosity’ covers a range of pore sizes and hydraulic conductivities in between fracture and
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Table 2 Calculation of % pesticide applied reaching the groundwater by preferential means in 1996–7 IPU
CTU
IPU
Effective porosity (%) Effective pore volume (v/v) Recharge through chalk matrix (mm) Maximum preferential recharge through chalk fractures (mm) % IPU/CTU applied reaching groundwater (max. spike conc.) % IPU/CTU applied reaching groundwater (avg. spike conc.)
100 0.43 490 162 0.07 0.05
0.06 0.06
0.1 0.07
Additional data used for calculations above
Br
IPU
CTU
Rainfall between coring (mm) Chalk porosity (v/v) Distance between Br peaks (m) Max. IPU/CTU spike conc. (mg/l) measured in 1996–7 (Johnson et al., 2001) Avg. IPU/CTU spike conc. (mg/l) in 1996–7
652.6 0.43 1.14 0.54 0.4
0.8 0.72
CTU
IPU
85 0.37 417 236
CTU 50 0.22 245 408
0.09 0.09
0.17 0.13
0.16 0.15
IPU applied at 1.3 kgha21 on 13th December 1995. CTU applied at 2.0 kgha21 on 30th October 1996.
matrix porosities. A possible conceptual explanation of these intermediate porosities or storage sites is given in Section 4.5. The postulated effect of the capillary fringe above the water table on the intermediate porosity is demonstrated in Fig. 7. Under this scheme, recharge
to deep groundwaters in the chalk aquifer is restricted to flow through the chalk matrix only. Recharge fluxes through the upper chalk horizons will be little impacted upon by the deep groundwater; only the fine matrix porosity (Fig. 7) will be kept water-filled up to at least 30 m above the water table (Price et al.,
Fig. 7. Schematic representation of the different hydrological condition between the deep and shallow groundwater sites resulting from the capillary fringe effect.
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Fig. 8. Conceptual description of the development of ‘intermediate’ storage/porosity as explained by ‘contact point’ theory.
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2000). Therefore, although the matrix is saturated, the intermediate and fracture porosities will be empty. Water flow will initially recharge through the fine matrix porosity. Only when the hydraulic conductivity of the matrix is exceeded will the intermediate porosity fill, and subsequently only when the hydraulic conductivity of the intermediate porosity is exceeded will the vertical fractures begin operating, first as thin-film flow and then as full fracture flow. Recharge drainage fluxes, which might move quickly through the upper chalk horizons, will be attenuated as the intermediate storage sites are filled. Not only will there be a delay in the travel time down the profile of peak drainage flux following peak rainfall, but this will also result in an overall reduction in drainage flux down the profile. So, although some vertical preferential flow in the chalk horizons closest to the soil occurs, the attenuation in water flux at depth means the hydraulic conductivity of the chalk matrix is sufficient to transmit the recharging water. Consequently, the fracture porosity does not need to operate. At shallow groundwater sites (e.g. dry valley bottoms) the impact of the capillary fringe on the chalk profile above the water table is far more significant. Fig. 7 shows how, in addition to the matrix porosity, the intermediate porosity may be largely water-filled resulting in reduced storage sites for vertical drainage fluxes in response to rainfall. This means that the unsaturated profile will quickly wet up in response to vertical drainage fluxes to a point where both the matrix porosity and intermediate porosity are water-filled. Any further drainage fluxes, which exceed the hydraulic conductivity of the chalk matrix and intermediate porosity, will subsequently move rapidly to the groundwater along fracture pathways. 4.5. Intermediate chalk storage porosity Hodnett and Bell (1990) described how the contact area within horizontally separated chalk peds would form a bottleneck in hydraulic connectivity and so would be the first loci to wet up and exhibit thin-film preferential behaviour. This localised wetting at contact points is schematically described in Fig. 8. Fig. 8(a) shows how at low drainage fluxes the hydraulic conductivity at the contact points is
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sufficient to transmit the water downward. As the drainage flux increases, the small contact area becomes restrictive to vertical water movement. Consequently, thin water films develop at the contact points (Fig. 8(b)) to accommodate the increased vertical water fluxes. These water films increase in thickness (Fig. 8(c)) to increase the hydraulic conductivity by increasing the cross-sectional waterfilled porosity thereby reducing tortuosity of flow pathways. When the horizontal fracture openings are water-filled, but vertical fractures are empty, the vertical water flux through the system will be equal to the maximum matrix conductivity. Any further increase in drainage flux will invoke vertical thin film and fracture flow pathways. Hence, ‘contact point’ theory can describe the generation of effective storage sites, in a dual porosity system, that are ‘intermediate’ between the fine matrix porosity and the much greater fracture pore sizes. Since ‘contact point’ theory explains ‘intermediate’ storage as the partial filling of fractures, this storage is very difficult to quantify using established water release characteristic and pore size distribution measurements. Recently, Price et al. (2000) described water storage sites in the irregularities on chalk fracture surfaces as an additional storage component to the traditional chalk dual porosity concept. Along with micro-fractures, this surface storage can also represent a pore size that is intermediate to the fracture and matrix porosities in addition to the ‘contact point’ storage described above.
5. Conclusions Results from this study show how groundwater recharge processes can be very different depending on the depth to the water table. Preferential fracture flow was observed at a shallow groundwater site (, 4 m depth), whilst for the same storm the recharge pathways, where the groundwater was deeper (, 18 m), were through the chalk matrix only. The different recharge processes observed resulted from the capillary fringe effect on ‘intermediate’ storage sites in the unsaturated zone above the water table. These ‘intermediate’ storage sites on chalk surfaces and at ‘contact points’ at the shallow groundwater site were largely water-filled and therefore had a reduced
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capacity for accommodating incoming water fluxes when compared to the deep groundwater sites. The shallow unsaturated profile rapidly reached potentials close to saturation resulting in preferential pathways operating at 3.0 m depth. Recharge fluxes where the groundwater was deeper were attenuated as they were ‘held’ and then slowly released from similar ‘intermediate’ storage sites. Consequently, during this study, the recharge fluxes at 3.0 m depth over a deep groundwater site never reached potentials where the hydraulic conductivity of the chalk matrix was exceeded, and so the conditions for preferential flow were never reached. Shallow groundwater sites are more likely to demonstrate preferential water and solute transport and therefore represent areas likely to pose the greatest risk of groundwater contamination. Bromide tracer analysis was used in an attempt to quantify the amount of pesticide that might travel by preferential routes to the groundwater at these sites. Calculations suggested that approximately 0.1% of the applied pesticide reached the groundwater preferentially in the 1996– 7 crop season. Worst-case estimates over the same period indicated no more than about 0.2% of the applied pesticide reached the groundwater by preferential flow paths at the shallow groundwater site in this study. This paper highlights the complex relationship between the groundwater and unsaturated zone hydrological processes. It is very difficult to differentiate and draw clear distinctions between the two zones with each impacting on the other in very intricate ways. The resulting processes of recharge and solute transport to shallow groundwater systems are thus poorly understood and require more research to further elucidate this important issue which has been shown, in this paper, to potentially enhance the contamination risk of a chalk aquifer.
Acknowledgements The authors wish to thank NERC for supporting this work. The authors also thank Andy Dixon for borehole installation and discussions on the hydrogeology of the site, and J.D. Cooper for his invaluable experience of chalk gleaned through countless discussions. The authors are grateful to the Instruments Section at CEH Wallingford for
their expertise and assistance in field instrumentation, and to Site Services for providing suitable support. Thanks are also given to the chemists at Wallingford for their analysis of the bromide samples and the British Geological Survey (BGS) for providing groundwater level data. Finally, the authors acknowledge the assistance given by the farm manager that was vital to the success of this study.
References Anonymous, 1995. Pesticides in the aquatic environment, Water Quality Series no. 26, Report of the National Rivers Authority, London: HMSO. Anonymous, 1999. Pesticides in the Aquatic Environment 1997. Environment Agency, Howbery Park, Wallingford, UK. Barraclough, D., Gardner, C.M.K., Wellings, S.R., Cooper, J.D., 1994. A tracer investigation into the importance of fissure flow in the unsaturated zone of the British Upper Chalk. J. Hydrol. 156, 459–469. Bell, J.P., 1976. Neutron probe practice. Natural Environment Research Council, Institute of Hydrology, Wallingford, Report no. 19. Besien, T.J., Williams, R.J., Johnson, A.C., 2000. The transport and behaviour of isoproturon in unsaturated chalk cores. J. Contam. Hydrol. 43, 91– 110. Butler, D.R., Huband, N.D.S., 1985. Throughfall and stem-flow in wheat. Agric. For. Meteorol. 35, 329–338. Cooper, J.D., Gardner, C.M.K., Mackenzie, N., 1990. Soil controls on recharge to aquifers. J. Soil Sci. 41, 613 –630. Dixon, A.J., 1989. An open drive method for borehole sampling sand and gravel deposits. Ground. Engng 22, 32– 35. Foster, S.S.D., 1993. The chalk aquifer—its vulnerability to pollution. In: Downing, R.A., Price, M., Jones, G.P. (Eds.), The hydrogeology of the Chalk of North-West Europe, Clarendon Press, Oxford, pp. 93 –112. Gillham, R.W., 1984. The capillary fringe and its effect on watertable response. J. Hydrol. 67, 307–324. Headworth, H.G., 1972. The analysis of natural groundwater level fluctuations in the chalk of Hampshire. J. Inst. Water Engng Sci. 26, 107–124. Headworth, H.G., 1989. Contamination of groundwaters from diffuse sources arising from farming activities. J. IWEM 3, 517–521. Hodnett, M.G., Bell, J.P., 1990. Processes of water movement through a chalk coombe deposit in southeast England. Hydrol. Process. 4, 361–372. Jarvis, M.G., Allen, R.H., Fordham, S.J., Hazelden, J., Moffat, A.J., Sturdy, R.G., 1984. Soils and their use in south east England. Soil Survey of England and Wales, Bulletin No. 15, Harpenden. Johnson, A.C., Hughes, C.D., Williams, R.J., Chilton, P.J., 1998. Potential for aerobic isoproturon biodegradation and sorption in
A.H. Haria et al. / Journal of Hydrology 275 (2003) 122–137 the unsaturated and saturated zones of a chalk aquifer. J. Contam. Hydrol. 30, 281–297. Johnson, A.C., Besien, T.J., Lal Bharadwaj, C., Dixon, A., Goody, D.C., Haria, A.H., White, C., 2001. Penetration of herbicides to groundwater in an unconfined chalk aquifer following normal soil applications. J. Contam. Hydrol. 53, 101–117. Kinniburgh, D.K., Miles, D.L., 1983. Extraction and chemical analysis of interstitial water from soils and rocks. Environ. Sci. Technol. 17, 362–368. Lloyd, J.W., 1993. The United Kingdom. In: Downing, R.A., Price, M., Jones, G.P. (Eds.), The Hydrogeology of the Chalk of North-West Europe, Clarendon Press, Oxford, pp. 93–112. Price, M., Bird, M.J., Foster, S.S.D., 1976. Chalk pore-size measurements and their significance. Water Services 80, 596–600. Price, M., Downing, R.A., Edmunds, W.M., 1993. The chalk as an aquifer. In: Downing, R.A., Price, M., Jones, G.P. (Eds.), The Hydrogeology of the Chalk of North-West Europe, Clarendon Press, Oxford, pp. 35– 58.
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Price, M., Low, R.G., McCann, C., 2000. Mechanisms of water storage and flow in the unsaturated zone of the Chalk aquifer. J. Hydrol. 233, 54–71. Thompson, N., Barrie, I.A., Ayles, M., 1981. The Meteorological Office Rainfall and Evaporation Systems: MORECS (July 1981). Met. Office Hydrological Memorandum No. 45. Tokunaga, T.K., Wan, J., 1997. Water film flow along fracture surfaces of porous rock. Water Resour. Res. 33 (6), 1287–1295. Tokunaga, T.K., Wan, J., 2001. Surface-zone flow along unsaturated rock fractures. Water Resour. Res. 37 (2), 287 –296. Wellings, S.R., 1984a. Recharge of the upper chalk aquifer at a site in Hampshire, England. 1. Water balance and unsaturated flow. J. Hydrol. 69, 259 –273. Wellings, S.R., 1984b. Recharge of the upper chalk aquifer at a site in Hampshire, England. 2. Solute movement. J. Hydrol. 69, 275 –285. Wellings, S.R., Bell, J.P., 1980. Movement of water and nitrate in the unsaturated zone of Upper Chalk near Winchester, Hants., England. J. Hydrol. 48, 119 –136.
Mechanisms of groundwater recharge and pesticide penetration to a chalk aquifer in southern England Atul H. Haria*, Martin G. Hodnett, Andrew C. Johnson Centre for Ecology and Hydrology (formerly Institute of Hydrology), Maclean Building, Crowmarsh Gifford, Wallingford, Oxfordshire OX10 8BB, UK Received 25 November 2002; accepted 7 January 2003
Abstract In order to assess the potential for enhanced agrochemical contamination of shallow groundwaters, a field study was established on the Upper Chalk in Hampshire, UK. Two instrumented sites, 380 m apart, were established on a hillslope; one on the interfluve with a deep water table (,18 m depth), the other on the dry valley bottom where the groundwater was close to the surface (, 4 m depth). Hourly measurements of water potentials in the unsaturated zone to 3.0 m depth identified very different groundwater recharge processes between the two sites in response to the same storm event. On the interfluve site with the deep water table only matrix flow through the chalk unsaturated zone at 3 m depth was identified. In contrast, at the dry valley bottom with a shallow water table, both rapid preferential flow and matrix flow processes were observed at a 3 m depth. The correlation between groundwater depth measurements and unsaturated profile moisture content measurements demonstrated the importance of the capillary fringe in sustaining a higher moisture content in the unsaturated zone at the shallow groundwater site. The resulting reduced water storage capacity for vertical drainage fluxes meant that little water was required to wet the shallow profile before rapid preferential flow events, demonstrated by rapid water potential responses, occurred. However, where the groundwater was deeper, ‘intermediate’ storage sites located on chalk surfaces and at chalk ped/block ‘contact points’ remained empty and unsaturated water potential profiles showed that rainfall pulses were attenuated as these sites absorbed the downward water fluxes. Consequently, preferential events at these deep groundwater sites are rare. The importance of these ‘intermediate’ storage sites in controlling recharge processes is highlighted. The potential mass load of pesticide transported to the shallow groundwater in preferential events during 1996– 7 was determined using bromide tracer studies, water balance calculations and measured groundwater pesticide concentrations. An estimated 0.1% of the applied pesticide reached the shallow groundwater along preferential pathways in 1996– 7. Calculations under a worst-case scenario showed that this value did not increase beyond 0.2% of pesticide applied. q 2003 Elsevier Science B.V. All rights reserved. Keywords: Chalk; Groundwater recharge; Unsaturated zone; Hillslope hydrology; Capillary fringe; Isoproturon; Chlotoluron
1. Introduction
* Corresponding author. Fax: þ 44-(0)-1491-692424. E-mail address: [email protected] (A.H. Haria).
Chalk aquifers of south-east England contribute 55% of all groundwater use in the UK (Lloyd, 1993). Large tracts of the typically thin soils overlying these
0022-1694/03/$ - see front matter q 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-1694(03)00017-9
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unconfined aquifers are under arable land use and the water quality of this potable resource remains an important issue. The impact of intensive agriculture on underlying groundwater was first realised in the mid-1970s when observed nitrate concentrations exceeded European Union permissible limits (Foster, 1993). Concern in recent years has spread to pesticide contamination following the introduction of the 0.1 mg/l drinking water limit by the EU (Headworth, 1989; Anonymous, 1995, 1999; Johnson et al., 2001). Chalk has been described as a dual porosity system where water flow can occur both in the chalk matrix and through the fractures between the chalk blocks (Price et al., 1993). The Upper Chalk, a sub-division of UK chalk based on lithology (the others being Lower and Middle Chalk), has specific physical properties. The maximum pore diameter of the Upper Chalk matrix is about 1.0 mm, with an associated intergranular hydraulic conductivity of about 0.001 m/day (Price et al., 1976). The porosity and permeability of the Upper Chalk matrix is greater than that of the Middle and Lower Chalk. Understanding the mechanisms of groundwater recharge is critical to assessing pollutant transfer to the water table. If water and solutes move rapidly through fractures then dilution, attenuation and degradation will be minimised and the contaminant is likely to arrive at relatively high concentrations soon after application on the soil surface. However, if transport is through the matrix then the much slower travel time will allow for dispersion, dilution and potential degradation (Johnson et al., 1998) which will influence the final concentration that reaches the groundwater. In addition to fracture flow and matrix flow a third mechanism of thin film flow (i.e. water film flow along fracture surfaces of porous rocks where the fractures are largely unsaturated) needs to be considered. In fractured porous media at potentials close to saturation, Tokunaga and Wan (1997) showed how thin film flows could be significant in moving water at fluxes about three orders in magnitude greater than matrix pore water under unit gradient saturated flow. Recently, Tokunaga and Wan (2001) have identified ‘surface-zone flows’ as another fast flow system in the enhanced porosity of the rock skin at the fracture surface. Vertical fracture flow in chalk will only begin when the vertical drainage flux exceeds the saturated
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hydraulic conductivity of the chalk matrix (Wellings, 1984a; Cooper et al., 1990; Price et al., 1993). Wellings (1984a) observed fracture flow only once in a five year period in the unsaturated zone over a deep groundwater site on the Upper Chalk in Hampshire. Although fractures have the ability to conduct large volumes of water quickly to depth, fracture flow is rare and the predominant mode of recharge is through the fine pores of the chalk matrix. Wellings (1984a) showed that fracture flow was generated only when matric chalk water potentials exceed a threshold value of 2 5 kPa. In a study on the Upper Chalk of the Berkshire downs, Barraclough et al. (1994) showed tracer profiles (15NO3, 2H2O and Cl2) that were also inconsistent with significant vertical macrofissure flow, and were explained by flow through the water filled porosity of the chalk matrix. A number of field studies conducted on sites with a thick unsaturated zone over a deep water table have measured rates of water movement through the chalk matrix to be about 1 m/yr (Wellings and Bell, 1980; Wellings, 1984b; Barraclough et al., 1994). However, to date, little work on the hydrology of shallow chalk groundwater systems has been conducted. Gillham (1984) described the effect of the capillary fringe on shallow groundwater responses and the implications for contaminant transport. The work, however, refers to homogeneous systems and not a dual porosity system like chalk. Recent work on the Upper Chalk over a shallow water table has shown how these groundwaters may receive sudden peaks of high pesticide concentration shortly after heavy rainfall (Johnson et al., 2001). This would suggest that these shallow water table sites may represent ‘hot spots’ for rapid pesticide transport to the groundwater. This paper describes a field study that was designed to determine the differences, if any, in recharge mechanisms between deep and shallow groundwater sites and whether this may lead to a greater contamination risk where the groundwater is close to the soil surface. Using a combination of hydrological measurements and bromide tracer studies an assessment of groundwater recharge and pesticide transport was made. Water balance calculations combined with pesticide concentrations measured in groundwater following rainfall were used to estimate the pesticide load arriving rapidly to the shallow
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groundwaters. The hypothesis of a field having different recharge mechanisms in different areas and therefore different risk potentials for groundwater contamination was addressed.
2. Site description The study area WON (Johnson et al., 1998) was located on the Upper Chalk in Hampshire, about 24 km north of Winchester and 1 km from a small tributary of the River Test. The study was located on a hillslope in an arable field on the chalk outcrop and comprised of an upper interfluve site and, at the foot of the slope (4 –5% gradient), a lower dry valley floor site. The dry valley floor site, subsequently referred to as WON 4, had a water table that was approximately 4 m from the soil surface and the interfluve site, subsequently referred to as WON 7, had a water table approximately 18 m below ground level. The distance between sites WON 4 and WON 7 was about 380 m (Fig. 1). The top 0.8 m of the profile at WON 4 is a silty clay loam soil described as Andover series (Jarvis et al., 1984), with increasing amounts of chalk mixed in below this depth; the soil-free chalk begins between 1.5 and 2.0 m below the soil surface. Chalk gravels, created by cryoturbation and other weathering processes, are found to a 3.0 m depth; the whole weathered chalk profile extending to 5 m below the surface. Additionally, chalk with a putty type structure, likely to be weathered chalk sludge washed down from the higher slopes, was also present in some areas. Observations from a 3 m deep pit at WON 7 showed a soil profile (Andover Series; Jarvis et al., 1984) with an average thickness of 0.3 m overlying a weathered chalk horizon (0.3 –1.0 m) with isolated patches of chalk gravels. Fracturing was observed to decrease with depth. During the 1996– 7 agricultural crop season the field was sown with grass, however, 5 £ 5 m plots over the boreholes were sown with wheat to permit pesticide use. Further details on the site and cropping patterns can be found elsewhere (Johnson et al., 1998; 2001).
3. Methods 3.1. Hydrological monitoring 3.1.1. Site WON 4 Soil and chalk water contents were recorded weekly through the 1996– 7 season using a neutron probe (Bell, 1976) in two access tubes spaced 0.5 m apart to a depth of 6 m. Readings were taken at 0.1 m depth increments to 0.5 m and then at 0.25 m depth increments to 6 m. Soil and chalk water potentials were measured using Pressure Transducer Tensiometers (PTTs) installed vertically by auguring through access tubes to ensure a watertight seal, to depths of 0.1, 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0 m. The PTTs were installed in two rows 0.5 m apart with a similar spacing between each PTT within the rows. The PTT site was situated in the corner of the field approximately 15 m from the closest field boundary and 3 m upslope of a 150 mm diameter observation borehole, WON 4. Rainfall was measured using a tipping bucket gauge and data were logged hourly. In addition to on-site rainfall measurements, MORECS (Thompson et al., 1981) rainfall and evaporation data were used for water balance calculations providing information outside the instrumented field season; farming practice meant field instruments had to be removed for part of the year. The permanent borehole at WON 4 was drilled on 12th November 1991 to 8 m below ground level using a dry percussion drilling method (Dixon, 1989). Sufficient slotted PVC casing (78 mm i.d.) was installed to accommodate the seasonal water level fluctuations. The annulus adjacent to the slots was infilled with clean gravel, followed by a 0.5 m sealing layer of bentonite above, and cement grout to the surface. The depth of the water table below the ground surface was recorded hourly using a pressure transducer interfaced to a data logger. Further boreholes (e.g. WON 5) were drilled close to WON 4 to provide additional chemical data (Johnson et al., 2001). 3.1.2. Site WON 7 The borehole at WON 7 was drilled using the same dry percussion method on 8th September 1996 to a depth of 20.71 m. WON 7, 77 m above sea level and 382 m upslope of WON 4, represented the site with
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Fig. 1. Schematic representation of the study area showing the deep groundwater site WON 7 and the shallow groundwater site WON 4.
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the greatest depth of unsaturated zone to the water table (18 m approx.) (Fig. 1). Water potentials were also measured at this site using Pressure transducer tensiometers (PTTs) installed to the same depths and in the same manner as for site WON 4. Data were logged hourly. 3.2. Bromide tracer studies On 22nd February 1995, 8 l of 50,000 mg/l KBr tracer solution was applied manually using a watering can with a rose spout in a 24 m2 grid 1 m upslope from borehole WON 4. Chalk cores of about 0.25 m length were extracted (on 28th September 1996 and 22nd September 1997) using a dry percussion sampling method on the site of application to assess, over time, bromide transport through the chalk matrix. Pore water from the cores was extracted by centrifugation using the method described by Kinniburgh and Miles (1983) and analysed for Br2 by ion chromatography (Dionex model 2010i). The eluent used was 1.8 mM sodium carbonate and 1.7 mM sodium bicarbonate. The regenerate used was 25 mM sulphuric acid. Detection was by electrical conductivity. Pore water Br2 concentrations for the profile were obtained.
4. Results and discussion 4.1. Hillslope recharge processes PTT data describing recharge processes through the unsaturated chalk profile at sites WON 4 and WON 7 were compared for a single rainfall event in 1997 (Storm 1, Table 1 and Fig. 2). The comparison was restricted to one storm event because of practical difficulties experienced with PTTs at WON 7; potentials at this site were very low for most of the season, and often below the minimum potential that can be recorded by tensiometers (2 80 kPa). Only Table 1 Rainfall data for Storm 1 and Storm 2 Event
Date
Rainfall (mm)
Duration (hrs)
Avg. intensity (mm/hr)
Storm 1 Storm 2
23–27 Feb 97 3–4 Mar 97
38 14.5
22 19
1.7 0.8
after air was purged from the PTTs insitu, were reliable readings obtained for short periods. It was fortunate that the PTTs at WON 7 were purged (denoted by ‘P’ in Fig. 2(b)) immediately prior to Storm 1. Data in Fig. 2(b) prior to purging, ‘P’, were unreliable and should be ignored. The need for regular purging of tensiometers at the deep groundwater site (WON 7) is clear evidence of highly negative potentials and drier profile to a 3 m depth compared with the shallow groundwater site (WON 4) where the wetter profile meant that the PTTs worked efficiently for the season. Different responses in groundwater recharge, between WON 4 and WON 7 unsaturated chalk zones, were observed for Storm 1 (Fig. 2). Unsaturated hydraulic potentials (0.5 –3.0 m depth) at WON 4 increased in two phases on 13th February 1997 and 18th February 1997 in response to rainfall. The increases were simultaneous from 1.5 to 3.0 m depths, marked A and B in Fig. 2(c)), resulting from piston displacement of downward water fluxes through the uniform porosity of the chalk matrix. The low chalk water potentials (more obvious in the deeper profile) mean that flow is through the matrix porosity only. The rate of change of potential in the chalk was greatest nearest the soil surface indicating a wetting pulse moving down the profile (Fig. 2(c)). The overall increase in water potentials show the gradual wetting of the profile by small rainfall events prior to Storm 1. Under these antecedent conditions, the rainfall associated with Storm 1 was of sufficient volume and intensity to allow the water flux to be transmitted quickly through the profile. The sharp (short duration) peaks in hydraulic potential in response to Storm 1 (Fig 2(c)) indicate larger fluxes over a short interval, compared to the preceding period. The peaks showing highest potential indicate when the hydraulic conductivity was at its highest. Depending on the gradient, the greatest flux will then occur at almost the same time. Similar findings in the Middle Chalk have been observed by Hodnett and Bell (1990). The anomaly arrowed X in Fig. 2(c) shows an early rise in potential clearly visible at 3.0 m and less so at 2.5 m. These may be explained either by decreased barometric pressure as a frontal system associated with rainfall passed over, causing instantaneous hydrostatic pressure responses at the water table in the borehole, or by the weight of rainwater on
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Fig. 2. Time series hydraulic potential responses to Storm 1 showing the attenuated response at WON 7 compared to the fast response at WON 4.
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the whole aquifer system causing isostatic responses in the borehole. Similar water level fluctuations were cited by Headworth (1972), in a review of groundwater level fluctuations in the chalk of Hampshire. Assuming the highest water potentials represent the peak water flux through the profile, Fig. 2(b) shows a much greater delay and attenuation in transit of peak water fluxes at the deep groundwater site WON 7, in response to Storm 1, than the sharp peaks in hydraulic potential observed at WON 4 (Fig. 2(c)). Peak water fluxes reached a 3.0 m depth in just over a day at the shallow groundwater site WON 4, whilst at WON 7 the peak flux arrived seven days after peak rainfall (Fig. 3). The hydrological response at the shallow groundwater site WON 4 to Storm 1 is more clearly demonstrated in Fig. 4(b), which shows the changes in hydraulic potential with depth from 11th to 26th February 1997. Following rainfall, the potentials increase, moving towards the line denoting
Fig. 3. Graph showing the delay in peak drainage flux with depth, after peak rainfall, at WON 7 compared to WON 4.
gravitational potential (saturation) until they become parallel with it at an approximate matric potential of 2 2.4 kPa during peak flux at 3.0 m depth (26th February 1997). The increase in potential indicates an increase in water content and therefore hydraulic conductivity to a point where the increasing downward water fluxes can be transmitted through the profile. Under unit gradient conditions, where the hydraulic potential profile is parallel to the line denoting gravitational potential as at WON 4 (Fig. 4(b)), the increase in drainage flux is proportional to the increase in hydraulic conductivity. Previous work on conductivity– potential relationships (Wellings, 1984a; Hodnett and Bell, 1990; Cooper et al., 1990; Tokunaga and Wan, 1997) has shown that small increases in water potential close to saturation can greatly increase the hydraulic conductivity. In this case a matric potential of 2 2.4 kPa is well within the threshold figure of 2 5 kPa for the onset of preferential fracture flow in the Upper Chalk as determined by Wellings (1984a). Changes in hydraulic potential with depth at the deep groundwater site WON 7, between 26th February 1997 and 3rd March 1997 (in response to Storm 1), are shown in Fig. 4(a). Matric potentials at 3.0 m depth did not exceed 2 17 kPa (well below the 2 5 kPa threshold for the initiation of fracture flow) during peak drainage (3rd March 1997) at this depth, showing that water flow was through the chalk matrix only. However, in the upper profile matric potentials as high as 2 2 kPa were observed indicating preferential or thin-film flows occurring down to a 1.0 m depth which were subsequently attenuated deeper in the chalk (discussed later). This indicates a lag in the drainage flux as downward water fluxes are ‘held’ and slowly released. At the surface this is likely to be a result of hydrodynamic dispersion in the soil horizon, however, the lag is also evident well below the soil zone (e.g. 2 – 3 m depth). Conventional thinking cannot explain this phenomenon since at the low potentials in question the fractures would all be empty and the uniform matrix porosity would all be waterfilled. The results highlight the existence of potentially important ‘intermediate’ storage sites (between the fracture and matrix porosities) where downward water fluxes can be ‘held’ and consequently reduced. A very small attenuation in drainage flux is observed at the shallow groundwater site (WON 4) suggesting
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Fig. 4. Change in hydraulic potential with depth during peak drainage flux in response to Storm 1 at (a) WON 7 and (b) WON 4.
that the ‘intermediate’ storage sites are largely waterfilled, so reducing the storage capacity of the profile. By the time peak drainage fluxes reached a depth of 2 m and below at WON 7, chalk water potentials at 1.5 m and above were becoming more negative, indicating the upper profile had started to dry out (Fig. 4(a)). Hence, the rainfall event which caused preferential flow at 2– 3 m at the shallow groundwater site (WON 4) resulted in matrix flow only through the 2 –3 m chalk at the deep groundwater site (WON 7). This is despite a thicker soil layer at site WON 4,
compared with WON 7, which might be expected to absorb and hold more of the rainfall releasing it more slowly to the chalk below. This study shows that it is likely to be a reduction in the water storage capacity at WON 4, itself likely to be a result of the shallow groundwater, which is the most important factor in governing the occurrence of preferential flow events. The effect of Storm 2 (Table 1) for both sites had less of an impact on recharge (Fig. 2). A unit potential gradient had already been established at the WON 4, and Storm 2 caused very slight rises in potential at 1.5
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and 2.0 m depths with no effect observed below this depth. Similarly, very little response to Storm 2 was observed below 2.0 m depth at WON 7. This emphasises the importance of individual storm characteristics in which, regardless of the antecedent conditions, certain threshold criteria of rainfall intensity and volume still need to be met before significant preferential/thin film flows can occur. For both Storms 1 and 2, surface PTTs (0.1 m depth) showed conditions remained unsaturated indicating the absence of runoff at either site. There was also no discernable response of the groundwater level at WON 4 in response to Storm 1 or Storm 2. There was a steady rise in groundwater level from 5.7 m below ground level to 4.7 m below ground level over the period shown in Fig. 2 (9th February 1997– 9th March 1997). Groundwater levels over the 1996/7 season fluctuated from 4.5 m below ground level to greater than 6 m below ground level. 4.2. Shallow groundwater effects Neutron probe data from WON 4 in the 1996– 7 season provides evidence of the influence of changes in shallow groundwater level on storage in the unsaturated chalk profile above. Moisture content data for 3 layers in the unsaturated chalk above the water table (2 –3 m, 3 –4 m and 4– 5 m depth layers) show that the layer closest to the groundwater (4 –5 m depth) was consistently wetter than the 3– 4 m layer above, which in turn was wetter than the 2 –3 m layer highest up (Fig. 5; time series data not shown). Correlating the moisture content in the 3 chalk layers with groundwater depth showed the greatest influence of the groundwater, throughout the 1996– 7 season, on the unsaturated chalk profile closest to the water table (Fig. 5). The coefficient of determination (r 2 ) is greatest for the layer closest to the groundwater, decreasing towards the surface layers. Significance F testing of groundwater on unsaturated storage showed an increasing influence with depth: 2 –3 m layer (0.024), 3 – 4 m layer (, 0.01) and 4 – 5 m layer ( ! 0.01). These data suggest that in the 1996 – 7 season groundwater had a significant impact on unsaturated chalk water storage to a depth of 3– 4 m, and a lesser impact above that depth.
4.3. Groundwater recharge and pesticide penetration at WON 4 Information on mean pore water velocity of recharge through the chalk matrix at the shallow groundwater site WON 4, between September 1996 and September 1997, was derived from the differences in bromide peak travel between the profiles in Fig. 6. Recovery of bromide from profile 1 was 104%, however, the forward tailing suggests this value is likely to be greater because of incomplete recovery. Recovery from profile 2 was 63%. The variability in bromide application by hand combined with the effects of micro-topography and the small sampling volume (0.1 m (4 inch) diameter cores) indicate that little can be concluded from this information. Certainly, more replicate cores needed to be taken at any one sampling period. The distance moved by the peak for profile 1, calculated as 2.17 m (the mid-point of the flat plateau), to the peak for profile 2 (3.31 m) was 1.14 m. The recharge through the chalk matrix required to move bromide peak 1 to peak 2 (Table 2, Fig. 6) was calculated based on three effective water filled porosity values of 0.43 v/v (100%), 0.37 v/v (85%, after Besien et al., 2000) and 0.22 v/v (50%, after Barraclough et al., 1994). The latter represents the worst-case scenario, with the more realistic results based on an effective porosity of 0.37 v/v calculated from breakthrough experiments on cores taken from site WON 4 (Besien et al., 2000). The remaining rainfall was assumed to be partitioned between crop interception/evaporation, soil water storage changes and rapid preferential drainage. MORECS (Thompson et al., 1981) data indicate that there was only 88 mm of recharge (rainfall minus evaporation) between the times of the two cored profiles. With a water content of 0.37, this would explain a downward movement of bromide of only about 0.24 m, compared with the observed movement of 1.14 m. It is therefore concluded that the recharge must have been greater than calculated by MORECS, probably because some of the evaporative demand was satisfied by the shallow groundwater. Movement of shallow groundwater into the unsaturated profile above, along upward gradients during the summer months, was identified by PTT data (not shown).
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Fig. 5. Correlation between moisture content in three unsaturated chalk layers and groundwater depth at WON 4 showing the greatest effect of the shallow groundwater on the deepest stratum.
In order to estimate the potential pesticide mass likely to reach the groundwater along preferential pathways for the period between cored bromide profiles, water balance calculations were combined with data of pesticide concentrations found in the groundwater. The evaporation component of the water balance was assumed to be negligible because: (a) the groundwater inputs satisfied much of the evaporative demand in the summer, (b) preferential recharge was likely to occur over a short duration during the wet winter months when evaporation would be lowest. Although information on crop interception was limited, estimates of canopy interception for winter wheat range from 30 to 40% where the leaf area index (LAI) is greater than 1 (Butler and Huband, 1985). However, in the 1996 –7 season, the field had been sown with grass and the expected interception would be less. Water balance calculations where rainfall from June to August 1997 (when LAI is greater than 1) was reduced by 30% did not make
significant differences to the pesticide load reaching the groundwater. Therefore, to bias the estimated pollution risk by preferential flow events to a worst-case scenario, the interception was assumed to be negligible. For water balance purposes, changes in profile water storage were also assumed to be negligible over the year because of capillary rise from the shallow groundwater. Weekly neutron probe data also showed very small changes in the chalk profile water content during the experimental season. The volume of rainfall not travelling through the chalk matrix (identified by the bromide profiles) was, therefore, allocated to preferential recharge to provide a worst-case scenario (Table 2). The spiked concentrations of pesticides Isoproturon (IPU) and Chlorotoluron (CTU), recorded over 3 years in the groundwater close to the water table (borehole WON 5) at site WON 4 by automated daily sampling after rainfall (Johnson et al., 2001), were used as a measure of
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Fig. 6. Bromide concentration profiles in the chalk unsaturated zone at WON 4 in September 1996 and September 1997.
the pesticide concentration in the water moving in the fractures. Although there will be some degree of dilution, these samples represented the best available estimates of the mass balance of pesticide preferentially reaching the groundwater. Based on the values observed by Johnson et al. (2001), estimates of potential pesticide concentrations likely to reach the groundwater by preferential fracture flow during the 1996 – 7 season are presented in Table 2. The findings suggest that, with an effective porosity of 0.37 v/v (Besien et al., 2000) and averaged pesticide spike concentrations (Johnson et al., 2001), approximately 0.1% IPU or CTU applied, preferentially reached the groundwater in the 1996 –7 season. Calculations under a worst-case scenario show that no more than 0.2% of applied IPU and CTU reached the groundwater in the 1996 –97 crop season by preferential flow
mechanisms. These results provide a field measured estimate of rapid pesticide loading to a shallow unconfined chalk aquifer. 4.4. Conceptual model of groundwater recharge processes Although chalk is considered to be a dual porosity system, this study has identified the effect of a third ‘intermediate’ porosity or storage site as the main controlling factor in recharge processes to shallow groundwater systems. This intermediate porosity can be described as that which is greater in size than the Upper Chalk matrix porosity and smaller than the fractures that become water-filled at potentials greater than 2 5 kPa matric suction. Note that the term ‘intermediate porosity’ covers a range of pore sizes and hydraulic conductivities in between fracture and
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Table 2 Calculation of % pesticide applied reaching the groundwater by preferential means in 1996–7 IPU
CTU
IPU
Effective porosity (%) Effective pore volume (v/v) Recharge through chalk matrix (mm) Maximum preferential recharge through chalk fractures (mm) % IPU/CTU applied reaching groundwater (max. spike conc.) % IPU/CTU applied reaching groundwater (avg. spike conc.)
100 0.43 490 162 0.07 0.05
0.06 0.06
0.1 0.07
Additional data used for calculations above
Br
IPU
CTU
Rainfall between coring (mm) Chalk porosity (v/v) Distance between Br peaks (m) Max. IPU/CTU spike conc. (mg/l) measured in 1996–7 (Johnson et al., 2001) Avg. IPU/CTU spike conc. (mg/l) in 1996–7
652.6 0.43 1.14 0.54 0.4
0.8 0.72
CTU
IPU
85 0.37 417 236
CTU 50 0.22 245 408
0.09 0.09
0.17 0.13
0.16 0.15
IPU applied at 1.3 kgha21 on 13th December 1995. CTU applied at 2.0 kgha21 on 30th October 1996.
matrix porosities. A possible conceptual explanation of these intermediate porosities or storage sites is given in Section 4.5. The postulated effect of the capillary fringe above the water table on the intermediate porosity is demonstrated in Fig. 7. Under this scheme, recharge
to deep groundwaters in the chalk aquifer is restricted to flow through the chalk matrix only. Recharge fluxes through the upper chalk horizons will be little impacted upon by the deep groundwater; only the fine matrix porosity (Fig. 7) will be kept water-filled up to at least 30 m above the water table (Price et al.,
Fig. 7. Schematic representation of the different hydrological condition between the deep and shallow groundwater sites resulting from the capillary fringe effect.
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Fig. 8. Conceptual description of the development of ‘intermediate’ storage/porosity as explained by ‘contact point’ theory.
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2000). Therefore, although the matrix is saturated, the intermediate and fracture porosities will be empty. Water flow will initially recharge through the fine matrix porosity. Only when the hydraulic conductivity of the matrix is exceeded will the intermediate porosity fill, and subsequently only when the hydraulic conductivity of the intermediate porosity is exceeded will the vertical fractures begin operating, first as thin-film flow and then as full fracture flow. Recharge drainage fluxes, which might move quickly through the upper chalk horizons, will be attenuated as the intermediate storage sites are filled. Not only will there be a delay in the travel time down the profile of peak drainage flux following peak rainfall, but this will also result in an overall reduction in drainage flux down the profile. So, although some vertical preferential flow in the chalk horizons closest to the soil occurs, the attenuation in water flux at depth means the hydraulic conductivity of the chalk matrix is sufficient to transmit the recharging water. Consequently, the fracture porosity does not need to operate. At shallow groundwater sites (e.g. dry valley bottoms) the impact of the capillary fringe on the chalk profile above the water table is far more significant. Fig. 7 shows how, in addition to the matrix porosity, the intermediate porosity may be largely water-filled resulting in reduced storage sites for vertical drainage fluxes in response to rainfall. This means that the unsaturated profile will quickly wet up in response to vertical drainage fluxes to a point where both the matrix porosity and intermediate porosity are water-filled. Any further drainage fluxes, which exceed the hydraulic conductivity of the chalk matrix and intermediate porosity, will subsequently move rapidly to the groundwater along fracture pathways. 4.5. Intermediate chalk storage porosity Hodnett and Bell (1990) described how the contact area within horizontally separated chalk peds would form a bottleneck in hydraulic connectivity and so would be the first loci to wet up and exhibit thin-film preferential behaviour. This localised wetting at contact points is schematically described in Fig. 8. Fig. 8(a) shows how at low drainage fluxes the hydraulic conductivity at the contact points is
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sufficient to transmit the water downward. As the drainage flux increases, the small contact area becomes restrictive to vertical water movement. Consequently, thin water films develop at the contact points (Fig. 8(b)) to accommodate the increased vertical water fluxes. These water films increase in thickness (Fig. 8(c)) to increase the hydraulic conductivity by increasing the cross-sectional waterfilled porosity thereby reducing tortuosity of flow pathways. When the horizontal fracture openings are water-filled, but vertical fractures are empty, the vertical water flux through the system will be equal to the maximum matrix conductivity. Any further increase in drainage flux will invoke vertical thin film and fracture flow pathways. Hence, ‘contact point’ theory can describe the generation of effective storage sites, in a dual porosity system, that are ‘intermediate’ between the fine matrix porosity and the much greater fracture pore sizes. Since ‘contact point’ theory explains ‘intermediate’ storage as the partial filling of fractures, this storage is very difficult to quantify using established water release characteristic and pore size distribution measurements. Recently, Price et al. (2000) described water storage sites in the irregularities on chalk fracture surfaces as an additional storage component to the traditional chalk dual porosity concept. Along with micro-fractures, this surface storage can also represent a pore size that is intermediate to the fracture and matrix porosities in addition to the ‘contact point’ storage described above.
5. Conclusions Results from this study show how groundwater recharge processes can be very different depending on the depth to the water table. Preferential fracture flow was observed at a shallow groundwater site (, 4 m depth), whilst for the same storm the recharge pathways, where the groundwater was deeper (, 18 m), were through the chalk matrix only. The different recharge processes observed resulted from the capillary fringe effect on ‘intermediate’ storage sites in the unsaturated zone above the water table. These ‘intermediate’ storage sites on chalk surfaces and at ‘contact points’ at the shallow groundwater site were largely water-filled and therefore had a reduced
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capacity for accommodating incoming water fluxes when compared to the deep groundwater sites. The shallow unsaturated profile rapidly reached potentials close to saturation resulting in preferential pathways operating at 3.0 m depth. Recharge fluxes where the groundwater was deeper were attenuated as they were ‘held’ and then slowly released from similar ‘intermediate’ storage sites. Consequently, during this study, the recharge fluxes at 3.0 m depth over a deep groundwater site never reached potentials where the hydraulic conductivity of the chalk matrix was exceeded, and so the conditions for preferential flow were never reached. Shallow groundwater sites are more likely to demonstrate preferential water and solute transport and therefore represent areas likely to pose the greatest risk of groundwater contamination. Bromide tracer analysis was used in an attempt to quantify the amount of pesticide that might travel by preferential routes to the groundwater at these sites. Calculations suggested that approximately 0.1% of the applied pesticide reached the groundwater preferentially in the 1996– 7 crop season. Worst-case estimates over the same period indicated no more than about 0.2% of the applied pesticide reached the groundwater by preferential flow paths at the shallow groundwater site in this study. This paper highlights the complex relationship between the groundwater and unsaturated zone hydrological processes. It is very difficult to differentiate and draw clear distinctions between the two zones with each impacting on the other in very intricate ways. The resulting processes of recharge and solute transport to shallow groundwater systems are thus poorly understood and require more research to further elucidate this important issue which has been shown, in this paper, to potentially enhance the contamination risk of a chalk aquifer.
Acknowledgements The authors wish to thank NERC for supporting this work. The authors also thank Andy Dixon for borehole installation and discussions on the hydrogeology of the site, and J.D. Cooper for his invaluable experience of chalk gleaned through countless discussions. The authors are grateful to the Instruments Section at CEH Wallingford for
their expertise and assistance in field instrumentation, and to Site Services for providing suitable support. Thanks are also given to the chemists at Wallingford for their analysis of the bromide samples and the British Geological Survey (BGS) for providing groundwater level data. Finally, the authors acknowledge the assistance given by the farm manager that was vital to the success of this study.
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