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The impact of artificial dune infiltration on the nutrient content of ground and surface water
Biological Conservation 34 (1985) 149-167
The Impact of Artificial Dune Infiltration on the Nutrient Content of Ground and Surface Water
H. W. J. v a n D i j k Department of Environmental Biology, State University of Leiden, Post Box 9516, 2300 RA Leiden, The Netherlands
ABSTRACT Many coastal dune areas in the west oj' The Netherlands are being infiltrated artificially with eutrophic water Jor public water supply purposes. As large volumes are involved, this caused a eutrophication process in the whole dune areas concerned. The investigations described deal with the penetration of dissolved macro-nutrients into the dune ecosystem. A tracer study showed that the upper 2"5 m of the ground water were jar more affected by the infiltration than JollowedJrom a hypothesis of a separate water layer Jormed on top of the infiltrated water. In all three areas studied, potassium occurred in unnaturally high concentrations in ground and surface water up to 500 m away from the infiltration ponds. Transport of nitrate introduced with the infiltration water could be revealed only locally and incidentally, and within 100 m from infiltration ponds. Only one of the three dune areas studied showed large-scale transport of phosphate within seepage areas, up to several hundreds of metres away from the infiltration ponds. In this area, phosphate absorbing peat layers in the upper aquifer are absent. This is assumed to be the main factor jor the high phosphate concentrations observed here.
INTRODUCTION M a n y coastal dune areas in the west of The Netherlands have been infiltrated artificially since about 1955. This infiltration is carried out by 149 Biol. Conserv. 0006-3207/85/$03-30 L')ElsevierApplied Science Publishers Ltd, England, 1985. Printed in Great Britain
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H. W. J. van Dijk
dune water boards to store and purify surface water from elsewhere to meet drinking water standards. The infiltration waters are taken from severely polluted sources: the rivers Rhine and Meuse and polder watercourses. These waters are very eutrophic. In the dune areas involved, the amount of infiltrated water exceeds the natural recharge by a factor 10 to 20 (Bakker, 1981). Dune infiltration occurs from ponds of about 50 to 300 m in length and 15 to 50m in width. Their depth usually ranges between 1.5 to 2.5 (sometimes up to 7)m. Catchment points (drains or wells) are situated at a certain distance from the ponds so that the infiltrated water is held in the dunes for a period of 1 to 3 months. Seepage areas with many pools and marshes may occur between the infiltration ponds and the sea, or between the ponds and the catchment points. The seepage pools studied are situated at a distance of 10 to about 500 m from the infiltration ponds. They are much smaller and shallower, their depth usually being 0.5to l m . Dune infiltration causes a highly increased nutrient load in the areas concerned (Bakker, 1981), which consequently affects the dune vegetation (Van der Werf, 1974; Londo, 1975; Van der Meulen, 1982; Van Dijk, 1984). On the banks of infiltration ponds the vegetation has become dominated by extremely nitrophilous tall hemicryptophytes. However, the influence of dune infiltration goes beyond these banks. Nitrophilous plant species have also begun to dominate the margins of the seepage pools, even at great distances from the infiltration ponds. This is an obstacle for the re-establishment of the original wet dune slack vegetations with their many rare species. Originally it was assumed that a layer of precipitation water would cover the infiltration water in those seepage areas where infiltration water is held for a relatively long period. In that case, the ecosystem of the seepage areas would not be affected by the eutrophic infiltration water. The observation of an abundance of nitrophilous plants along the seepage pools causes one to doubt the assumption that the upper layer of the ground water remains unaffected. The present study tests the hypothesis of a so-called 'precipitation lens' in the seepage areas of infiltrated dune areas. Furthermore it examines whether macronutrients can be transported from the infiltration ponds to the seepage areas through the groundwater course. Primarily, this study was restricted to one area only, namely Meijendel. Later, two other infiltrated dune areas, Berkheide and Luchterdunes, were also studied for comparison.
151
Nutrients in dune water
G R O U N D WATER STUDY IN MEIJENDEL Research area and methods Meijendel is the dune area managed by the Dune Water Company of The Hague. Its location is presented in Fig. 1. Three sampling areas were selected in Meijendel (Fig. 2). The average horizontal velocity of the phreatic ground water in these areas is 0-03, 0.1 and 0.3mday -1 respectively. These velocities range between the natural (lower) velocity and the high velocity of infiltration water flowing from infiltration pond to catchment point (generally 1 to 1.Smday-Z).The velocities were derived from ground water level gradients using Darcy's law and were checked by studying the movement of infiltration water with a clear-cut decrease of chloride content (1976) between 1974 and 1980 (Van Dijk, 1984).The upper 2.5m of the ground water was sampled monthly at intervals of 0.5 m (i.e. 5 samples per sampling plot). Six sampling plots
~o
0
~:~:::: ,~:~D
° iI t
L /I iI
(// i
C~
/ 0
60kin
/
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Fig. 1. Coastal dunes (hatched) in The Netherlands and location of areas studied. a, Luchterdunes; b, Berkheide dunes; c, Meijendel dunes (l, Amsterdam; 2, The Hague).
152
H. W. J. van Dijk
-
-
--
coast-line
flowline A ',!,,6x5
:
flowline B
x4 i
i6x~-i
ii
~ ×
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1 i
! flowline C
500 m
Fig. 2. Survey of the three flowlines sampled in the Meijendel dunes.
were selected per area at distances from the infiltration ponds of between 10 and about 400 m. The analyses presented in this paper concern the tracer minerals chloride and potassium and the macro-nutrients orthophosphate, nitrate and, again, potassium. The methods of chemical analysis were as follows: Chloride:
titration with silver nitrate, visual end point indication on potassium chromate.
Nutrients in dune water
Potassium:
Orthophosphate:
Nitrate:
153
direct measuring with atomic absorption spectrophotometer (Perkin-Elmer 460). extinction-measurement after adding a m m o n i u m heptamolybdate and tin chloride (with PerkinElmer 35 spectrophotometer). extinction-measurement after adding sodium salicylate (Perkin-Elmer 35).
The fraction of infiltration water in the ground water
The concentration of the selected tracers is much higher in infiltration water than it is in precipitation water. The effects of biogenic and soilchemical processes concerning the tracer concentrations are considered to be negligible. The tracer-concentration in the infiltration water during penetration into the dune soil is a known factor and can be combined with the precipitation concentration, distance from infiltration ponds and velocity of ground water flow in order to calculate the relative mixing of the ground water and infiltration water per sampling plot. On the basis of hydrodynamic dispersion, a development of the tracer concentration as given in Fig. 3 may be expected. For two velocities of the ground water flow at each spot a gradient of the fraction of the infiltration water related with the depth of the ground water has been calculated. With a ground water velocity of 0.1 m day - 1 at a distance of about 50 m from the infiltration pond the expectation is that the fraction of infiltration water ranges from less than 20~o to 60-70 °~ /o at depths increasing from nil to 2.5 m below the ground water table. With a velocity of 0.3 m d a y - 1 the same gradient is expected at a distance of about 200 m at a depth increasing from nil to over 3 m below the water table. However, it appears that the presupposed gradient of the fraction of infiltration water occurs in weak forms in only 2 out of the 18 sampling plots. In these two plots the concentrations in the ground water roughly increase from precipitation values to infiltration water values within 2 m under the ground water table. In other plots a strong mixing of the upper ground water layers was observed, much stronger than might be expected on the grounds of the dispersion coefficient of pure dune sand which was used for the hypothesis given in Fig. 3. De Groot (1984) states that the mixing of the upper 2.5 m of the ground water is caused by significant irregularities in the dune soil, such as humus layers covered by sand, roots of shrubs, etc. The results of the ground
A2
A,
A~ A~
c/co-_o.~ C/Co:O3\ C/Co:O.1 C/Co=0.6 C/Co:0.4 C/Co=0.2
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3,
100m
\
B2
o
C/Co=0.9
~
C/Co:O.8 = 0
B3
.
B,
C/Co:0.7
6
B~
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%
=02
Fig. 3. Theoretical rainwater/infiltration water mixing pattern. Calculation as a transversal dispersion with ~'T = 0"05 m superimposed on the theoretical interface for flowlines A and B. C and C Odenote actual and infiltration water tracer concentrations, respectively. The average C/C 0 over a sampling plot compares with the empirical fraction of infiltration water (see Fig. 4). Source: De Groot (1984).
C/Co:o 7
i
A,
U~
155
Nutrients in dune water velocity ground water Different streamlines: A x . . . . . 100
0.1
m/day
¸
L, + . . . .
\,
0.03 m/day
v
~
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X
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x
o
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16o
260
36o
460
distance from infiltration p o n d s (m)
Fig. 4. Averagefraction of infiltrationwater in upper 2.5 m of ground water in relation to the distance from infiltration ponds (derived from annual average potassium concentrations). water analysis clearly show that the average fraction of infiltration water in the upper 2.5 m decreases as the retention time of the ground water increases. In other words, the lower the velocity of the ground water flow and the greater the distance from the infiltration lake, the less infiltration water is found in the upper 2.5 m of the ground water (see Fig. 4).
The growth-determining macro-nutrients in the ground water The macro-nutrients in the ground water will behave differently from the tracers. In the case of an ideal tracer dilution with precipitation water is the only factor which determines the concentration relative to the input value. In the case of macro-nutrients other mechanisms are at work: biogenic influences of vegetation and soil (micro) organisms and physicochemical influences of the soil. The biogenic influences will be highest for those macro-nutrients which are relatively growth limiting. For dune soils these concern nitrogen, phosphate and, to a lesser degree, potassium (Willis, 1963; Olssen, 1974). Nitrogen or phosphate are growth-determining in infiltration water. In the case of Meijendel, phosphate is the limiting macro-nutrient (Van Dijk, 1984). The results of the study of the macro-nutrients are compared with the behaviour of the tracers, the expectation on the basis of dilution with precipitation water. Nitrogen
More than 95 ~ of the mineral nitrogen content in the upper 2.5 m of the
156
H. W. J. van Dijk
ground water is in the form of nitrate in all cases. Seasonal fluctuations in concentration in the infiltrated water could be retraced in some ground water sampling plots in the vicinity of infiltration ponds. These fluctuations give an indication of the origin of the ground water. The nitrate which is supplied by the infiltration water can be observed clearly only within 100 m from infiltration ponds. This may be due to denitrification as a result of the presence of oxidizable organic compounds in the peat layers or in the infiltrated water. However, concentrations which were significantly higher than those in the infiltration water were found locally at distances of several hundreds of metres from the infiltration pond. There is no straightforward explanation for this phenomenon. It is likely that biological sources such as bird colonies and shrubs (nitrifying sea buckthorn) have some impact here.
Phosphate Only the concentration of free orthophosphate in the ground water was measured. This is roughly equivalent to the total phosphate content in the water concerned. Similar to the nitrate concentrations, the orthophosphate concentrations did not show a gradient of a decrease with increasing distance from infiltration ponds as found for the tracers. Both nitrate and orthophosphate behave differently. However, the spread of the individual phosphate values is so great that significant deviations from the values expected on the basis of the tracer behaviour and the input concentrations cannot be detected. The spread is mainly caused by relatively high values in spring and summer. Places which are not shown to be influenced by infiltration water in the tracer study (see Fig. 4) show annual average orthophosphate concentrations of 0-03-0.16 mg PO4a- litre-1 in the upper 2.5 m of phreatic ground water. Locally (i.e. close to the infiltration bank of the two courses with the highest velocity), the ground water mixed with infiltration water could show concentrations which may be much higher than the concentration in the infiltrated water, which had an annual average concentration of up to 0.32mg PO~-litre -1. This phenomenon is attributed to the mobilisation of phosphate ions which are fixed in the soil by adsorption and released into the ground water with a phosphate concentration which was lowered by improved pre-purification of the infiltration water. The Department of Environmental Biology thoroughly investigated
Nutrients in dune water
157
the possible relations between the salient fluctuation of the phosphate concentration in the ground water and oxygen content, temperature, water table fluctuations and iron content, but this study did not solve the problem. An acceptable explanation may be found in biological activity (G. C. Janze, pers. comm.).
SURFACE WATER IN SEEPAGE AREAS
Research areas, pools sampled and methods of analysis The study had to include the seepage areas of Berkheide and Luchterdunes because the low concentrations of phosphate in Meijendel made it almost impossible to quantify the ground water transport of phosphate here. Figure 1 presents the location of these two extra research areas. It is plausible to assume that a higher rate of nutrient transport occurs in Berkheide because of the more abundant growth of extremely nitrophilous tall hemicryptophytes on the banks of seepage pools in this area (Van Dijk, 1984). Because of practical obstacles the study of the three infiltration areas was limited to the analysis of the surface water of infiltration ponds and seepage pools and did not include a ground water analysis. In Berkheide and Luchterdunes most of the seepage pools are situated between the infiltration ponds and the catchment points; this is in contrast to Meijendel. Consequently, the ground water velocity in these two areas is generally much higher than in Meijendel. The selection requirements of the pools sampled included a maximum depth of 1.5 m. With this maximum depth it may be assumed that the ground water flow into the pools is restricted to the upper 2.5 m, which has been shown to be a mixture of infiltration water and precipitation water in the tracer study carried out in Meijendel. In Luchterdunes, Berkheide and Meijendel a total of 8, 23 and 12 seepage pools were selected with varying ground water velocities and varying distances from the infiltration ponds. In addition, samples were taken from 2, 7 and 5 infiltration ponds respectively at the beginning of the ground water courses studied. Seepage pools and infiltration ponds were sampled monthly for at least one year, some for 2 to 5 years. The chemical analyses of the water samples were mainly focused on potassium, nitrate and orthophosphate. The analysing methods applied
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H. W. J. van Dijk
are as given in the section on research area and methods at Meijendel. Most of the results are based on samples taken in 1976, 1977 and 1978. The results are given in the form of annual average concentrations. Table 1 presents an interpretation of the results and classifies the pools studied according to ground water velocity and distance from the nearest infiltration pond. The annual average concentrations are presented per group of pools. Annual average concentrations of the infiltration ponds in the vicinity are also presented for the same year and the year preceding pre-purification of the infiltration water. Pre-purification was improved significantly in the Luchterdunes in 1974 and in Meijendel in 1976. Results for potassium Pools which are not affected by artificial infiltration show average potassium concentrations of 1 to 2 (maximum 3) mg litre- 1 (Van Dijk & Meltzer, 1981). Compared with these values, the seepage pools of Berkheide and Meijendel contain an unnaturally high potassium concentration up to a distance of 500 m from the infiltration ponds. As was to be expected from the results of the ground water study in Meijendel, in all areas the natural values are exceeded to a lesser degree, as the ground water velocity in the seepage areas decreases and as the distance from the supplying infiltration pond increases. Results for nitrate The nitrate concentration gives a representative picture of the concentration of the total nitrogen content in the seepage pools, although nitrate in the surface water appears to be less important than it is in ground water. The highest nitrate concentrations were found in Luchterdunes. This is also the only area in which a gradient of decreasing concentration with increasing distance from the infiltration pond could be discovered. The fact that this gradient was not found in Berkheide nor in Meijendel indicates that the process of dilution with precipitation water is dominated by other physico-chemical or biological processes occurring in these areas. In the case of the nitrates, the main influence on the nitrate concentration should be sought in such biological processes as uptake by the vegetation and denitrification. However, these processes cannot prevent the fact that the natural nitrate concentrations (0-0.5 mg litre- 1
Nutrients in dune water
159
after Van Dijk & Meltzer, 1981) are generally exceeded in Meijendel and Berkheide. In most of the seepage pools which have been sampled for 5 years (all those in Meijendel), marked increases in the nitrate concentration have been observed in the course of time. Some increases in annual average concentrations between successive years amounted to 300 ~o or more. Discussion of nitrate
De Groot & Steenkamp (1979) state that in the course of 20 years considerable increases in the nitrogen concentration occurred in the catchment water of all three infiltration areas studied. The nitrogen concentration increased much more than can be accounted for by the increased concentration in infiltration water. Short-term studies of seepage pools in Meijendel (maximum 6 years) show a clear tendency towards increasing nitrogen loads in most pools (Van Dijk, 1984). An explanation for both observations may be found in a decrease in the rate of denitrification in the ground water as a result of reduced sources of organic compounds. Two possible reasons for this may be given: in the first place 'exhaustion' of the easily oxidisable peat, and second by the decreasing concentration of organic compounds in the infiltrated water due to improved pre-purification. For the increased nitrogen concentration in the seepage pool water a totally different explanation may be found in an increasing impact of natural nitrogen sources. Two of the possible sources in Meijendel have increased significantly during the past few years: the herring-gull Larus argentatus colony (Wanders, 1982) and the shrubs of the nitrifying sea buckthorn Hippophak" rhamnoides. Observations of Stuyfzand (1984) on the transfer of nitrogen in another dune area verify the above assumption for dense stands of sea buckthorn. It is not possible to give a single explanation for the occurrence of a much higher nitrogen concentration in the seepage pools of Luchterduinen compared with the other two areas studied. One of the causes may be found in the fact that the water which is brought into the Luchterdunes shows the highest nitrogen concentration. Another explanation may be that pre-purification of organic compounds from the infiltration water has been applied more effectively and for a longer period than in the other two areas. This might have a negative effect on the denitrification process in the soil.
Seepage Seepage Seepage Seepage
pools, pools, pools, pools,
high high high high
Infiltration ponds
velocity velocity velocity velocity
Berkheide 1977-1978
Seepage pools Seepage pools Seepage pools
Infiltration ponds
Luchterdunes 1976
Area~Type oJ'pond or pool
TABLE 1
15-20 70-140 185~30 240-300
0
20-40 50 130 c
0
Distance from connected infiltration pond (m)
3 (4) 4 (7) 3 (4) 3 (4)
7
3 3b 2
2
Number a o f ponds or pools sampled
0"98 0"81 0"29 0.15
(0'54) (0"66) (0-14) (0"06)
1'14 (0"35)
0.16 (0.14) 0-04 (0'02) 0"09 (0"06)
0"06 (0.02) a
Orthophosphate
0-9 1.2 2.3 0.5
(0"2) (0"6) (1"4) (0"2)
6.9 (3"9)
7"2 (3"4) 5-1 (1.7) 1.9 (2"6)
20.8 (8-5)
Nitrate
11-9 (1"7) 11.2 (2-8) 8-1 (3"6) 8'3 (4.5)
13"6 (1-4)
9 9 9
7 to 10
Potassium
Average o f annual average concentrations (mg litre (Standard deviation per class o f distance added in brackets)
Annual Average Macro-nutrient Concentrations in Seepage Pools
1)
u,
2 (2 3) 1
46-80 305 345
Seepage pools, low velocity Seepage pools, low velocity
0.06 (0.003) 0.07 (--)
0.12 (0.02) 0.07 (0'015) 0.08 (0.000)
0.25 (0.23)
0.37 (0.27) 0.20 (0.15) 0.12 (0-05)
0.6 (0.8) 0.2 (--)
1-4 (1'6) 2.4 (16) 1-5 (1.0)
10.0 (22)
1.4 (0.9) 1.2 (0.6) 0-8 (0.3)
3.2 (0.8) 1-3 (--)
4.8 (06) 2.5 (1"2) 2.0 (0.5)
5.1 (0"4)
8.7 (4.4) 4.2 (1-3) 3.2 (1.7)
QThe number of known annual average concentrations is given in brackets if ponds are sampled during two years. b One seepage pond is not included on account of an extra high influence of evaporation. c No inflow from infiltration ponds. d Concentration in infiltration ponds before improved purification: Luchterdunes 0-70mg PO 3 litre-1 (1974); Meijendel dunes 0.50mgPO]- L I T R E - 1 (1975).
4 (4~5) 3 (3-5) 2 (2)
46-80 153-235 305-345
Seepage pools, high velocity Seepage pools, high velocity Seepage pools, high velocity
4 (5) 3 (5) 3 (5) 5 (6-10)
20-30 32-60 65-130 0
Meijendel 1977-1978 Infiltration ponds
Seepage pools, low velocity Seepage pools, low velocity Seepage pools, low velocity
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H. W. J. van Dijk
Results for orthophosphate Because of the absence of intensive algal blooms in the seepage pools, orthophosphate gives a representative reflection of the total phosphate load. The 'available' orthophosphate concentrations are undoubtedly highest in the seepage pools of Berkheide. The lowest concentrations are found in the seepage pools of Meijendel. The concentrations in Meijendel hardly exceed the concentrations of non-infiltrated dune areas (0-0.05 mg phosphate litre-1 after Van Dijk & Meltzer (1981)). The higher concentration expected in the vicinity of the infiltration ponds does not occur in Meijendel. However, the expected significant negative relation between distance and concentration is found for the other two areas. In Berkheide, the decreasing concentration with increasing distance from the infiltration ponds is more significant than could be expected on the basis of dilution only. Absorption by the soil and, to a lesser degree, by the vegetation give possible explanations for the extra decrease during the ground water flow. In relation to the above it must be added that, between 1976 and 1978, a great number of seepage pools in Berkheide which were situated at a distance of 100-300 m from the infiltration lakes showed a considerable increase in the orthophosphate concentration, whereas the seepage pools in the vicinity of the infiltration lakes showed a fairly constant and extremely high concentration. These increases, which amount to several hundred percent, occurred at distances of 100-200 m from the (supplying) infiltration pond in the case of a low velocity and at distances of 200-300 m in the case of high velocities (see Fig. 5). Thus, the gradient of a decreasing phosphate concentration with increasing distance from the infiltration ponds becomes weaker.
Discussion of phosphate A saturation of absorption points in the soils concerned might explain the rapidly increasing phosphate concentrations. During the research period in Berkheide this saturation might have occurred at distances of 100-200 m from the infiltration lake with a low velocity and at a distance of 200-300 m with a high velocity. In this case the increased mobilisation of soil-fixed phosphate will have caused an increase in the phosphate supply to the seepage pools. If this assumption is right, then the pronounced gradient of high to low phosphate concentrations will
163
Nutrients in dune water
3-
velocity of ground
~ ]E ~ 0.5~ 0.2~ 0.1-
8O.06velocity of ground water below 0.1 m/day
o
16o
= -- 1976/1977 c-----o 1977/1978
2oo
360
406
distance from infiltrationponds
Fig. 5. Annualaverageorthophosphateconcentrationin seepagepools in relationto the distance from infiltration ponds during two successive years (Berkheide dunes, two flowlines). gradually disappear with a high ground water velocity. With an extremely low velocity the gradient will probably remain intact, even after a long period because of the effect of dilution with precipitation water. The seepage pool concentrations are highest in Berkheide. This may partly be due to the absence of peat layers in the relatively thin upper aquifer in this area (De Groot, 1981). As mentioned above, it was assumed that the relatively high abundance of extremely nitrophilous tall hemicryptophytes on the banks of seepage pools in Berkheide is caused by a relatively high nutrient supply in this area. Furthermore, Berkheide is also different from the other two areas in its relatively high infiltration water concentrations. If the hypothesis of Van Dijk (1984), which assumes that the phosphate supply is the dominant factor for the occurrence of nitrophilous tall hemicryptophytes in infiltrated dune areas, is right, then the abundance of these species in Berkheide can mainly be related to the absence of peat in the upper aquifer. The fact that the phosphate concentration of the infiltrated water and the ground water velocity in the seepage areas are relatively high seems to have less impact on the present vegetation. RELATIONS BETWEEN THE NUTRIENTS Dune infiltration not only affects the nitrate and phosphate concentrations as such, but it also affects the so-called N: P ratio of the surface and ground water. This ratio, which is the quotient of mineral nitrogen
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H. W. J. van Dijk
and orthophosphate, may be used to indicate which of the nutrients is growth-determining or limiting. With a ratio higher than 16-20 phosphate is growth-determining, with lower ratios nitrogen is growthdetermining for phytoplankton (Schmidt-Van Dorp, 1978). It is striking that the N: P ratios in the seepage pools of Luchterdunes and Meijendel, where phosphate in the infiltrated water is growthdetermining, are increased considerably in relation to the infiltration water, whereas in Berkheide the N: P ratio of the seepage pools appears to be much lower than in the infiltration ponds. This indicates that in seepage areas the existing growth-determining effect of nutrients in infiltration water has been pronounced. The N : P ratio may also explain why the expected gradient of a decreasing nutrient concentration with an increasing distance from the infiltration lakes mainly concerns the phosphate concentration in Berkheide, whereas it concerns the nitrogen concentration in Luchterduinen. In Berkheide, nitrogen is obviously the main growth-determining factor in relation to phosphate, whereas phosphate is the dominating growth-determining factor in Luchterdunes. The vegetation and plankton will absorb growth-limiting nutrients to such an extent that original gradients for the most extreme growthdetermining nutrients will be nullified. In that case, a gradient of a decreasing concentration with an increasing distance from the infiltration ponds, as found with the tracer study, can only be retraced for nutrients which are hardly growth-determining and which do not react with the soil. For this reason the macro-nutrient potassium could be used as an inert tracer. IMPLICATIONS FOR NATURE CONSERVATION AND MANAGEMENT It may be concluded from the above that dephosphorization of infiltration water offers better perspectives in controlling the nitrophilous tall hemicryptophytes than a reduction of the nitrogen or potassium concentrations. There appear to be natural sources of nitrogen in the dunes, which dominate the impact of infiltration, even with unpurified infiltration water. Potassium is less growth-determining than nitrate or orthophosphate. The fact that dephosphorization is a relatively uncomplicated process is a happy coincidence. This step in prepurification is already being applied by water companies, mainly because
Nutrients in dune water
165
it prevents excessive algal blooms and consequently improves the permeability of the bottom and bank soils of the infiltration ponds. Technically, dephosphorization can reduce the phosphate concentration to a level lower than the natural balance of the ground water in Meijendel (0-02-0"03 mg PO 3- litre-x according to Van Oosterhoud et al., 1982). Observations of ground water which flows from infiltration ponds to catchment points have shown that a significant reduction of the phosphate concentration in the infiltration water rapidly results in a marked decrease in the ground water concentration (Van Dijk, 1982). However, it is difficult to give a quantitative prediction of the behaviour of phosphate after improved pre-purification of the infiltration water, especially in the case of soils which are rich in peat layers. What does this imply for the management of the vegetation? As mentioned above, dephosphorization can reduce the orthophosphate concentration in the infiltration water below the natural value. However, Van Dijk (1984) observed that the occurrence of nitrophilous tall hemicryptophytes in infiltrated dune areas is determined mainly by the 'external phosphate load' (i.e. the product of the orthophosphate concentration and the ground water velocity), whereas the concentration alone is of minor importance. Artificial surface infiltration will always result in unnaturally high ground water velocities (1 to 1.5 m day- 1 in the vicinity of infiltration ponds and 0.03 to I mday-1 in seepage areas), whereas the ground water velocity ranges from 0.03 to 0.2 today- ~ under natural circumstances (Bakker, 1981). Therefore, surface infiltration will always foster species with a high nutrient demand at the expense of species belonging to the rich original dune slack vegetation, even with strong prepurification. The effect of pre-purification on the vegetation on the banks of infiltration ponds may, at the most, result in controlling the most extreme nitrophilous tall hemicryptophtes, such as stinging nettle Urtica dioica, great willowherb Epilobium hirsutum and hemp agrimony Eupatorium cannabinum in favour of less extreme nitrophilous tall hemicryptophytes, such as bush grass Calamagrostis epigeios and gipsywort Lycopus europaeus (Van Dijk, 1984). The impact of pre-purification on the vegetation of the seepage pools requires further research. ACKN OWLEDGE MENTS I am indebted to the following organisations for their stimulating co-operation: the Dune Water Company of The Hague, the Municipal
166
H. W. J. van Dijk
Water C o m p a n y o f Amsterdam, the D u n e Water C o m p a n y of Leiden and the Hugo de Vries-Laboratory (Department of Vegetation Studies and Plant Ecology) in Amsterdam. Mrs M. J. van Hezewijk, D. de Jonge, G. C. Janze, R. Joostensz van IJsseldijk, G. van Ommering, Mrs S. Osseman, P. Otte, P. van 't Sant, H. van der Weijer and S: van der Zwan gathered m a n y of the presented data. W. T. de Groot assisted in the processing o f the data and the interpretation o f the results and T. W. M. Bakker and Mrs A. R. Kaal made critical comments on the draft,
REFERENCES Bakker, T. W. M. (1981). Nederlandse kustduinen; Geohydrologie. PhD thesis, University of Agricultural Sciences, Wageningen. De Groot, W. T. (1981). Het gedrag van fosfaat bij duininfiltratie. H20, 14, 152-8.
De Groot, W. T. (1984). Hydrodynamic macro-dispersion as the causal background of the phreatic water type mixing in infiltrated dunes. In: Invloeden van oppervlakte-infiltratie ten behoeve van duinwaterwinning op kruidachtige oevervegetaties, ed. by H. W. J. van Dijk, 205-10. PhD thesis, University of Agricultural Sciences, Wageningen. De Groot, W. T. & Steenkamp, F. E. M. (1979). Nutrifintenbelasting van infiltratiegebieden. In: Waterwinning in de duinen," aantasting en regeneratiemogel(/kheden, 25-66. Centre for Environmental Sciences, State University of Leyden. Londo, G. (1975). Infiltreren is nivelleren. Levende Natuur, 78, 74-9. Olssen, H. (1974). Studies on South Swedish sand vegetation. Acta Phytogeografica Suecica, 60. Schmidt-Van Dorp, A. D. (1978). Eutrofie'ring van ondiepe meren in Rijnland (Holland). PhD thesis, State University of Utrecht. Stuyfzand, P. J. (1984). Effecten van vegetatie en luchtverontreiniging op de grondwaterkwaliteit in kalkrijke duinen bij Castricum: lysimeterwaarnemingen. H20, 17, 152-60. Van der Meulen, F. (1982). Vegetation changes and water catchment in a Dutch coastal dune area. Biol. Conserv., 24, 305-16. Van der Weft, S. (1974). Infiltratie: met water meer plant. In: Meijendel, duinwater-leven, ed. by N. Croin Michielsen, 226-30. The Hague, Baarn. Van Dijk, H. W. J. (1982). Invloeden van waterwinning door duininfiltratie op de vegetatie. Department of Environmental Biology, State University of Leyden. Van Dijk, H. W. J. (1984). Invloeden van oppervlakte-infiltratie ten behoeve van duinwaterwinning op kruidachtige oevervegetaties. PhD thesis, University of Agricultural Sciences, Wageningen.
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Van Dijk, H. W. J. & Meltzer, J. A. (1981). Hydrobiologie van natuurlijke duinmeren: een commentaar. H20, 14, 564-7. Van Oosterhoud, E., Janze, G. C., De Groot, W. T. & Van Dijk, H. W. J. (1982). Fosfaat en duininfiltratie: een experimentele benadering. H20, 15, 497-502. Wanders, E. A. J. (1982). Beheer van meeuwenkolonies in de Hollandse duinen. Duin, 5(2), 18-23. Willis, A. J. (1963). Braunton Burrows: The effects on the vegetation of the addition of mineral nutrients to the dune soils. J. Ecol., 51, 353-74.
The Impact of Artificial Dune Infiltration on the Nutrient Content of Ground and Surface Water
H. W. J. v a n D i j k Department of Environmental Biology, State University of Leiden, Post Box 9516, 2300 RA Leiden, The Netherlands
ABSTRACT Many coastal dune areas in the west oj' The Netherlands are being infiltrated artificially with eutrophic water Jor public water supply purposes. As large volumes are involved, this caused a eutrophication process in the whole dune areas concerned. The investigations described deal with the penetration of dissolved macro-nutrients into the dune ecosystem. A tracer study showed that the upper 2"5 m of the ground water were jar more affected by the infiltration than JollowedJrom a hypothesis of a separate water layer Jormed on top of the infiltrated water. In all three areas studied, potassium occurred in unnaturally high concentrations in ground and surface water up to 500 m away from the infiltration ponds. Transport of nitrate introduced with the infiltration water could be revealed only locally and incidentally, and within 100 m from infiltration ponds. Only one of the three dune areas studied showed large-scale transport of phosphate within seepage areas, up to several hundreds of metres away from the infiltration ponds. In this area, phosphate absorbing peat layers in the upper aquifer are absent. This is assumed to be the main factor jor the high phosphate concentrations observed here.
INTRODUCTION M a n y coastal dune areas in the west of The Netherlands have been infiltrated artificially since about 1955. This infiltration is carried out by 149 Biol. Conserv. 0006-3207/85/$03-30 L')ElsevierApplied Science Publishers Ltd, England, 1985. Printed in Great Britain
150
H. W. J. van Dijk
dune water boards to store and purify surface water from elsewhere to meet drinking water standards. The infiltration waters are taken from severely polluted sources: the rivers Rhine and Meuse and polder watercourses. These waters are very eutrophic. In the dune areas involved, the amount of infiltrated water exceeds the natural recharge by a factor 10 to 20 (Bakker, 1981). Dune infiltration occurs from ponds of about 50 to 300 m in length and 15 to 50m in width. Their depth usually ranges between 1.5 to 2.5 (sometimes up to 7)m. Catchment points (drains or wells) are situated at a certain distance from the ponds so that the infiltrated water is held in the dunes for a period of 1 to 3 months. Seepage areas with many pools and marshes may occur between the infiltration ponds and the sea, or between the ponds and the catchment points. The seepage pools studied are situated at a distance of 10 to about 500 m from the infiltration ponds. They are much smaller and shallower, their depth usually being 0.5to l m . Dune infiltration causes a highly increased nutrient load in the areas concerned (Bakker, 1981), which consequently affects the dune vegetation (Van der Werf, 1974; Londo, 1975; Van der Meulen, 1982; Van Dijk, 1984). On the banks of infiltration ponds the vegetation has become dominated by extremely nitrophilous tall hemicryptophytes. However, the influence of dune infiltration goes beyond these banks. Nitrophilous plant species have also begun to dominate the margins of the seepage pools, even at great distances from the infiltration ponds. This is an obstacle for the re-establishment of the original wet dune slack vegetations with their many rare species. Originally it was assumed that a layer of precipitation water would cover the infiltration water in those seepage areas where infiltration water is held for a relatively long period. In that case, the ecosystem of the seepage areas would not be affected by the eutrophic infiltration water. The observation of an abundance of nitrophilous plants along the seepage pools causes one to doubt the assumption that the upper layer of the ground water remains unaffected. The present study tests the hypothesis of a so-called 'precipitation lens' in the seepage areas of infiltrated dune areas. Furthermore it examines whether macronutrients can be transported from the infiltration ponds to the seepage areas through the groundwater course. Primarily, this study was restricted to one area only, namely Meijendel. Later, two other infiltrated dune areas, Berkheide and Luchterdunes, were also studied for comparison.
151
Nutrients in dune water
G R O U N D WATER STUDY IN MEIJENDEL Research area and methods Meijendel is the dune area managed by the Dune Water Company of The Hague. Its location is presented in Fig. 1. Three sampling areas were selected in Meijendel (Fig. 2). The average horizontal velocity of the phreatic ground water in these areas is 0-03, 0.1 and 0.3mday -1 respectively. These velocities range between the natural (lower) velocity and the high velocity of infiltration water flowing from infiltration pond to catchment point (generally 1 to 1.Smday-Z).The velocities were derived from ground water level gradients using Darcy's law and were checked by studying the movement of infiltration water with a clear-cut decrease of chloride content (1976) between 1974 and 1980 (Van Dijk, 1984).The upper 2.5m of the ground water was sampled monthly at intervals of 0.5 m (i.e. 5 samples per sampling plot). Six sampling plots
~o
0
~:~:::: ,~:~D
° iI t
L /I iI
(// i
C~
/ 0
60kin
/
~k
Fig. 1. Coastal dunes (hatched) in The Netherlands and location of areas studied. a, Luchterdunes; b, Berkheide dunes; c, Meijendel dunes (l, Amsterdam; 2, The Hague).
152
H. W. J. van Dijk
-
-
--
coast-line
flowline A ',!,,6x5
:
flowline B
x4 i
i6x~-i
ii
~ ×
~i~ 4 ! ×5
seepage pool
!
infiltration pond
:i~6
drainage system series of boreholes
1 i
! flowline C
500 m
Fig. 2. Survey of the three flowlines sampled in the Meijendel dunes.
were selected per area at distances from the infiltration ponds of between 10 and about 400 m. The analyses presented in this paper concern the tracer minerals chloride and potassium and the macro-nutrients orthophosphate, nitrate and, again, potassium. The methods of chemical analysis were as follows: Chloride:
titration with silver nitrate, visual end point indication on potassium chromate.
Nutrients in dune water
Potassium:
Orthophosphate:
Nitrate:
153
direct measuring with atomic absorption spectrophotometer (Perkin-Elmer 460). extinction-measurement after adding a m m o n i u m heptamolybdate and tin chloride (with PerkinElmer 35 spectrophotometer). extinction-measurement after adding sodium salicylate (Perkin-Elmer 35).
The fraction of infiltration water in the ground water
The concentration of the selected tracers is much higher in infiltration water than it is in precipitation water. The effects of biogenic and soilchemical processes concerning the tracer concentrations are considered to be negligible. The tracer-concentration in the infiltration water during penetration into the dune soil is a known factor and can be combined with the precipitation concentration, distance from infiltration ponds and velocity of ground water flow in order to calculate the relative mixing of the ground water and infiltration water per sampling plot. On the basis of hydrodynamic dispersion, a development of the tracer concentration as given in Fig. 3 may be expected. For two velocities of the ground water flow at each spot a gradient of the fraction of the infiltration water related with the depth of the ground water has been calculated. With a ground water velocity of 0.1 m day - 1 at a distance of about 50 m from the infiltration pond the expectation is that the fraction of infiltration water ranges from less than 20~o to 60-70 °~ /o at depths increasing from nil to 2.5 m below the ground water table. With a velocity of 0.3 m d a y - 1 the same gradient is expected at a distance of about 200 m at a depth increasing from nil to over 3 m below the water table. However, it appears that the presupposed gradient of the fraction of infiltration water occurs in weak forms in only 2 out of the 18 sampling plots. In these two plots the concentrations in the ground water roughly increase from precipitation values to infiltration water values within 2 m under the ground water table. In other plots a strong mixing of the upper ground water layers was observed, much stronger than might be expected on the grounds of the dispersion coefficient of pure dune sand which was used for the hypothesis given in Fig. 3. De Groot (1984) states that the mixing of the upper 2.5 m of the ground water is caused by significant irregularities in the dune soil, such as humus layers covered by sand, roots of shrubs, etc. The results of the ground
A2
A,
A~ A~
c/co-_o.~ C/Co:O3\ C/Co:O.1 C/Co=0.6 C/Co:0.4 C/Co=0.2
A3
scale
3,
100m
\
B2
o
C/Co=0.9
~
C/Co:O.8 = 0
B3
.
B,
C/Co:0.7
6
B~
~v
%
=02
Fig. 3. Theoretical rainwater/infiltration water mixing pattern. Calculation as a transversal dispersion with ~'T = 0"05 m superimposed on the theoretical interface for flowlines A and B. C and C Odenote actual and infiltration water tracer concentrations, respectively. The average C/C 0 over a sampling plot compares with the empirical fraction of infiltration water (see Fig. 4). Source: De Groot (1984).
C/Co:o 7
i
A,
U~
155
Nutrients in dune water velocity ground water Different streamlines: A x . . . . . 100
0.1
m/day
¸
L, + . . . .
\,
0.03 m/day
v
~
x
\ -\
X
._~ 5 0
c
\
"5
~o o
+
"~'\ ~
.
x
o
x~--
16o
260
36o
460
distance from infiltration p o n d s (m)
Fig. 4. Averagefraction of infiltrationwater in upper 2.5 m of ground water in relation to the distance from infiltration ponds (derived from annual average potassium concentrations). water analysis clearly show that the average fraction of infiltration water in the upper 2.5 m decreases as the retention time of the ground water increases. In other words, the lower the velocity of the ground water flow and the greater the distance from the infiltration lake, the less infiltration water is found in the upper 2.5 m of the ground water (see Fig. 4).
The growth-determining macro-nutrients in the ground water The macro-nutrients in the ground water will behave differently from the tracers. In the case of an ideal tracer dilution with precipitation water is the only factor which determines the concentration relative to the input value. In the case of macro-nutrients other mechanisms are at work: biogenic influences of vegetation and soil (micro) organisms and physicochemical influences of the soil. The biogenic influences will be highest for those macro-nutrients which are relatively growth limiting. For dune soils these concern nitrogen, phosphate and, to a lesser degree, potassium (Willis, 1963; Olssen, 1974). Nitrogen or phosphate are growth-determining in infiltration water. In the case of Meijendel, phosphate is the limiting macro-nutrient (Van Dijk, 1984). The results of the study of the macro-nutrients are compared with the behaviour of the tracers, the expectation on the basis of dilution with precipitation water. Nitrogen
More than 95 ~ of the mineral nitrogen content in the upper 2.5 m of the
156
H. W. J. van Dijk
ground water is in the form of nitrate in all cases. Seasonal fluctuations in concentration in the infiltrated water could be retraced in some ground water sampling plots in the vicinity of infiltration ponds. These fluctuations give an indication of the origin of the ground water. The nitrate which is supplied by the infiltration water can be observed clearly only within 100 m from infiltration ponds. This may be due to denitrification as a result of the presence of oxidizable organic compounds in the peat layers or in the infiltrated water. However, concentrations which were significantly higher than those in the infiltration water were found locally at distances of several hundreds of metres from the infiltration pond. There is no straightforward explanation for this phenomenon. It is likely that biological sources such as bird colonies and shrubs (nitrifying sea buckthorn) have some impact here.
Phosphate Only the concentration of free orthophosphate in the ground water was measured. This is roughly equivalent to the total phosphate content in the water concerned. Similar to the nitrate concentrations, the orthophosphate concentrations did not show a gradient of a decrease with increasing distance from infiltration ponds as found for the tracers. Both nitrate and orthophosphate behave differently. However, the spread of the individual phosphate values is so great that significant deviations from the values expected on the basis of the tracer behaviour and the input concentrations cannot be detected. The spread is mainly caused by relatively high values in spring and summer. Places which are not shown to be influenced by infiltration water in the tracer study (see Fig. 4) show annual average orthophosphate concentrations of 0-03-0.16 mg PO4a- litre-1 in the upper 2.5 m of phreatic ground water. Locally (i.e. close to the infiltration bank of the two courses with the highest velocity), the ground water mixed with infiltration water could show concentrations which may be much higher than the concentration in the infiltrated water, which had an annual average concentration of up to 0.32mg PO~-litre -1. This phenomenon is attributed to the mobilisation of phosphate ions which are fixed in the soil by adsorption and released into the ground water with a phosphate concentration which was lowered by improved pre-purification of the infiltration water. The Department of Environmental Biology thoroughly investigated
Nutrients in dune water
157
the possible relations between the salient fluctuation of the phosphate concentration in the ground water and oxygen content, temperature, water table fluctuations and iron content, but this study did not solve the problem. An acceptable explanation may be found in biological activity (G. C. Janze, pers. comm.).
SURFACE WATER IN SEEPAGE AREAS
Research areas, pools sampled and methods of analysis The study had to include the seepage areas of Berkheide and Luchterdunes because the low concentrations of phosphate in Meijendel made it almost impossible to quantify the ground water transport of phosphate here. Figure 1 presents the location of these two extra research areas. It is plausible to assume that a higher rate of nutrient transport occurs in Berkheide because of the more abundant growth of extremely nitrophilous tall hemicryptophytes on the banks of seepage pools in this area (Van Dijk, 1984). Because of practical obstacles the study of the three infiltration areas was limited to the analysis of the surface water of infiltration ponds and seepage pools and did not include a ground water analysis. In Berkheide and Luchterdunes most of the seepage pools are situated between the infiltration ponds and the catchment points; this is in contrast to Meijendel. Consequently, the ground water velocity in these two areas is generally much higher than in Meijendel. The selection requirements of the pools sampled included a maximum depth of 1.5 m. With this maximum depth it may be assumed that the ground water flow into the pools is restricted to the upper 2.5 m, which has been shown to be a mixture of infiltration water and precipitation water in the tracer study carried out in Meijendel. In Luchterdunes, Berkheide and Meijendel a total of 8, 23 and 12 seepage pools were selected with varying ground water velocities and varying distances from the infiltration ponds. In addition, samples were taken from 2, 7 and 5 infiltration ponds respectively at the beginning of the ground water courses studied. Seepage pools and infiltration ponds were sampled monthly for at least one year, some for 2 to 5 years. The chemical analyses of the water samples were mainly focused on potassium, nitrate and orthophosphate. The analysing methods applied
158
H. W. J. van Dijk
are as given in the section on research area and methods at Meijendel. Most of the results are based on samples taken in 1976, 1977 and 1978. The results are given in the form of annual average concentrations. Table 1 presents an interpretation of the results and classifies the pools studied according to ground water velocity and distance from the nearest infiltration pond. The annual average concentrations are presented per group of pools. Annual average concentrations of the infiltration ponds in the vicinity are also presented for the same year and the year preceding pre-purification of the infiltration water. Pre-purification was improved significantly in the Luchterdunes in 1974 and in Meijendel in 1976. Results for potassium Pools which are not affected by artificial infiltration show average potassium concentrations of 1 to 2 (maximum 3) mg litre- 1 (Van Dijk & Meltzer, 1981). Compared with these values, the seepage pools of Berkheide and Meijendel contain an unnaturally high potassium concentration up to a distance of 500 m from the infiltration ponds. As was to be expected from the results of the ground water study in Meijendel, in all areas the natural values are exceeded to a lesser degree, as the ground water velocity in the seepage areas decreases and as the distance from the supplying infiltration pond increases. Results for nitrate The nitrate concentration gives a representative picture of the concentration of the total nitrogen content in the seepage pools, although nitrate in the surface water appears to be less important than it is in ground water. The highest nitrate concentrations were found in Luchterdunes. This is also the only area in which a gradient of decreasing concentration with increasing distance from the infiltration pond could be discovered. The fact that this gradient was not found in Berkheide nor in Meijendel indicates that the process of dilution with precipitation water is dominated by other physico-chemical or biological processes occurring in these areas. In the case of the nitrates, the main influence on the nitrate concentration should be sought in such biological processes as uptake by the vegetation and denitrification. However, these processes cannot prevent the fact that the natural nitrate concentrations (0-0.5 mg litre- 1
Nutrients in dune water
159
after Van Dijk & Meltzer, 1981) are generally exceeded in Meijendel and Berkheide. In most of the seepage pools which have been sampled for 5 years (all those in Meijendel), marked increases in the nitrate concentration have been observed in the course of time. Some increases in annual average concentrations between successive years amounted to 300 ~o or more. Discussion of nitrate
De Groot & Steenkamp (1979) state that in the course of 20 years considerable increases in the nitrogen concentration occurred in the catchment water of all three infiltration areas studied. The nitrogen concentration increased much more than can be accounted for by the increased concentration in infiltration water. Short-term studies of seepage pools in Meijendel (maximum 6 years) show a clear tendency towards increasing nitrogen loads in most pools (Van Dijk, 1984). An explanation for both observations may be found in a decrease in the rate of denitrification in the ground water as a result of reduced sources of organic compounds. Two possible reasons for this may be given: in the first place 'exhaustion' of the easily oxidisable peat, and second by the decreasing concentration of organic compounds in the infiltrated water due to improved pre-purification. For the increased nitrogen concentration in the seepage pool water a totally different explanation may be found in an increasing impact of natural nitrogen sources. Two of the possible sources in Meijendel have increased significantly during the past few years: the herring-gull Larus argentatus colony (Wanders, 1982) and the shrubs of the nitrifying sea buckthorn Hippophak" rhamnoides. Observations of Stuyfzand (1984) on the transfer of nitrogen in another dune area verify the above assumption for dense stands of sea buckthorn. It is not possible to give a single explanation for the occurrence of a much higher nitrogen concentration in the seepage pools of Luchterduinen compared with the other two areas studied. One of the causes may be found in the fact that the water which is brought into the Luchterdunes shows the highest nitrogen concentration. Another explanation may be that pre-purification of organic compounds from the infiltration water has been applied more effectively and for a longer period than in the other two areas. This might have a negative effect on the denitrification process in the soil.
Seepage Seepage Seepage Seepage
pools, pools, pools, pools,
high high high high
Infiltration ponds
velocity velocity velocity velocity
Berkheide 1977-1978
Seepage pools Seepage pools Seepage pools
Infiltration ponds
Luchterdunes 1976
Area~Type oJ'pond or pool
TABLE 1
15-20 70-140 185~30 240-300
0
20-40 50 130 c
0
Distance from connected infiltration pond (m)
3 (4) 4 (7) 3 (4) 3 (4)
7
3 3b 2
2
Number a o f ponds or pools sampled
0"98 0"81 0"29 0.15
(0'54) (0"66) (0-14) (0"06)
1'14 (0"35)
0.16 (0.14) 0-04 (0'02) 0"09 (0"06)
0"06 (0.02) a
Orthophosphate
0-9 1.2 2.3 0.5
(0"2) (0"6) (1"4) (0"2)
6.9 (3"9)
7"2 (3"4) 5-1 (1.7) 1.9 (2"6)
20.8 (8-5)
Nitrate
11-9 (1"7) 11.2 (2-8) 8-1 (3"6) 8'3 (4.5)
13"6 (1-4)
9 9 9
7 to 10
Potassium
Average o f annual average concentrations (mg litre (Standard deviation per class o f distance added in brackets)
Annual Average Macro-nutrient Concentrations in Seepage Pools
1)
u,
2 (2 3) 1
46-80 305 345
Seepage pools, low velocity Seepage pools, low velocity
0.06 (0.003) 0.07 (--)
0.12 (0.02) 0.07 (0'015) 0.08 (0.000)
0.25 (0.23)
0.37 (0.27) 0.20 (0.15) 0.12 (0-05)
0.6 (0.8) 0.2 (--)
1-4 (1'6) 2.4 (16) 1-5 (1.0)
10.0 (22)
1.4 (0.9) 1.2 (0.6) 0-8 (0.3)
3.2 (0.8) 1-3 (--)
4.8 (06) 2.5 (1"2) 2.0 (0.5)
5.1 (0"4)
8.7 (4.4) 4.2 (1-3) 3.2 (1.7)
QThe number of known annual average concentrations is given in brackets if ponds are sampled during two years. b One seepage pond is not included on account of an extra high influence of evaporation. c No inflow from infiltration ponds. d Concentration in infiltration ponds before improved purification: Luchterdunes 0-70mg PO 3 litre-1 (1974); Meijendel dunes 0.50mgPO]- L I T R E - 1 (1975).
4 (4~5) 3 (3-5) 2 (2)
46-80 153-235 305-345
Seepage pools, high velocity Seepage pools, high velocity Seepage pools, high velocity
4 (5) 3 (5) 3 (5) 5 (6-10)
20-30 32-60 65-130 0
Meijendel 1977-1978 Infiltration ponds
Seepage pools, low velocity Seepage pools, low velocity Seepage pools, low velocity
162
H. W. J. van Dijk
Results for orthophosphate Because of the absence of intensive algal blooms in the seepage pools, orthophosphate gives a representative reflection of the total phosphate load. The 'available' orthophosphate concentrations are undoubtedly highest in the seepage pools of Berkheide. The lowest concentrations are found in the seepage pools of Meijendel. The concentrations in Meijendel hardly exceed the concentrations of non-infiltrated dune areas (0-0.05 mg phosphate litre-1 after Van Dijk & Meltzer (1981)). The higher concentration expected in the vicinity of the infiltration ponds does not occur in Meijendel. However, the expected significant negative relation between distance and concentration is found for the other two areas. In Berkheide, the decreasing concentration with increasing distance from the infiltration ponds is more significant than could be expected on the basis of dilution only. Absorption by the soil and, to a lesser degree, by the vegetation give possible explanations for the extra decrease during the ground water flow. In relation to the above it must be added that, between 1976 and 1978, a great number of seepage pools in Berkheide which were situated at a distance of 100-300 m from the infiltration lakes showed a considerable increase in the orthophosphate concentration, whereas the seepage pools in the vicinity of the infiltration lakes showed a fairly constant and extremely high concentration. These increases, which amount to several hundred percent, occurred at distances of 100-200 m from the (supplying) infiltration pond in the case of a low velocity and at distances of 200-300 m in the case of high velocities (see Fig. 5). Thus, the gradient of a decreasing phosphate concentration with increasing distance from the infiltration ponds becomes weaker.
Discussion of phosphate A saturation of absorption points in the soils concerned might explain the rapidly increasing phosphate concentrations. During the research period in Berkheide this saturation might have occurred at distances of 100-200 m from the infiltration lake with a low velocity and at a distance of 200-300 m with a high velocity. In this case the increased mobilisation of soil-fixed phosphate will have caused an increase in the phosphate supply to the seepage pools. If this assumption is right, then the pronounced gradient of high to low phosphate concentrations will
163
Nutrients in dune water
3-
velocity of ground
~ ]E ~ 0.5~ 0.2~ 0.1-
8O.06velocity of ground water below 0.1 m/day
o
16o
= -- 1976/1977 c-----o 1977/1978
2oo
360
406
distance from infiltrationponds
Fig. 5. Annualaverageorthophosphateconcentrationin seepagepools in relationto the distance from infiltration ponds during two successive years (Berkheide dunes, two flowlines). gradually disappear with a high ground water velocity. With an extremely low velocity the gradient will probably remain intact, even after a long period because of the effect of dilution with precipitation water. The seepage pool concentrations are highest in Berkheide. This may partly be due to the absence of peat layers in the relatively thin upper aquifer in this area (De Groot, 1981). As mentioned above, it was assumed that the relatively high abundance of extremely nitrophilous tall hemicryptophytes on the banks of seepage pools in Berkheide is caused by a relatively high nutrient supply in this area. Furthermore, Berkheide is also different from the other two areas in its relatively high infiltration water concentrations. If the hypothesis of Van Dijk (1984), which assumes that the phosphate supply is the dominant factor for the occurrence of nitrophilous tall hemicryptophytes in infiltrated dune areas, is right, then the abundance of these species in Berkheide can mainly be related to the absence of peat in the upper aquifer. The fact that the phosphate concentration of the infiltrated water and the ground water velocity in the seepage areas are relatively high seems to have less impact on the present vegetation. RELATIONS BETWEEN THE NUTRIENTS Dune infiltration not only affects the nitrate and phosphate concentrations as such, but it also affects the so-called N: P ratio of the surface and ground water. This ratio, which is the quotient of mineral nitrogen
164
H. W. J. van Dijk
and orthophosphate, may be used to indicate which of the nutrients is growth-determining or limiting. With a ratio higher than 16-20 phosphate is growth-determining, with lower ratios nitrogen is growthdetermining for phytoplankton (Schmidt-Van Dorp, 1978). It is striking that the N: P ratios in the seepage pools of Luchterdunes and Meijendel, where phosphate in the infiltrated water is growthdetermining, are increased considerably in relation to the infiltration water, whereas in Berkheide the N: P ratio of the seepage pools appears to be much lower than in the infiltration ponds. This indicates that in seepage areas the existing growth-determining effect of nutrients in infiltration water has been pronounced. The N : P ratio may also explain why the expected gradient of a decreasing nutrient concentration with an increasing distance from the infiltration lakes mainly concerns the phosphate concentration in Berkheide, whereas it concerns the nitrogen concentration in Luchterduinen. In Berkheide, nitrogen is obviously the main growth-determining factor in relation to phosphate, whereas phosphate is the dominating growth-determining factor in Luchterdunes. The vegetation and plankton will absorb growth-limiting nutrients to such an extent that original gradients for the most extreme growthdetermining nutrients will be nullified. In that case, a gradient of a decreasing concentration with an increasing distance from the infiltration ponds, as found with the tracer study, can only be retraced for nutrients which are hardly growth-determining and which do not react with the soil. For this reason the macro-nutrient potassium could be used as an inert tracer. IMPLICATIONS FOR NATURE CONSERVATION AND MANAGEMENT It may be concluded from the above that dephosphorization of infiltration water offers better perspectives in controlling the nitrophilous tall hemicryptophytes than a reduction of the nitrogen or potassium concentrations. There appear to be natural sources of nitrogen in the dunes, which dominate the impact of infiltration, even with unpurified infiltration water. Potassium is less growth-determining than nitrate or orthophosphate. The fact that dephosphorization is a relatively uncomplicated process is a happy coincidence. This step in prepurification is already being applied by water companies, mainly because
Nutrients in dune water
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it prevents excessive algal blooms and consequently improves the permeability of the bottom and bank soils of the infiltration ponds. Technically, dephosphorization can reduce the phosphate concentration to a level lower than the natural balance of the ground water in Meijendel (0-02-0"03 mg PO 3- litre-x according to Van Oosterhoud et al., 1982). Observations of ground water which flows from infiltration ponds to catchment points have shown that a significant reduction of the phosphate concentration in the infiltration water rapidly results in a marked decrease in the ground water concentration (Van Dijk, 1982). However, it is difficult to give a quantitative prediction of the behaviour of phosphate after improved pre-purification of the infiltration water, especially in the case of soils which are rich in peat layers. What does this imply for the management of the vegetation? As mentioned above, dephosphorization can reduce the orthophosphate concentration in the infiltration water below the natural value. However, Van Dijk (1984) observed that the occurrence of nitrophilous tall hemicryptophytes in infiltrated dune areas is determined mainly by the 'external phosphate load' (i.e. the product of the orthophosphate concentration and the ground water velocity), whereas the concentration alone is of minor importance. Artificial surface infiltration will always result in unnaturally high ground water velocities (1 to 1.5 m day- 1 in the vicinity of infiltration ponds and 0.03 to I mday-1 in seepage areas), whereas the ground water velocity ranges from 0.03 to 0.2 today- ~ under natural circumstances (Bakker, 1981). Therefore, surface infiltration will always foster species with a high nutrient demand at the expense of species belonging to the rich original dune slack vegetation, even with strong prepurification. The effect of pre-purification on the vegetation on the banks of infiltration ponds may, at the most, result in controlling the most extreme nitrophilous tall hemicryptophtes, such as stinging nettle Urtica dioica, great willowherb Epilobium hirsutum and hemp agrimony Eupatorium cannabinum in favour of less extreme nitrophilous tall hemicryptophytes, such as bush grass Calamagrostis epigeios and gipsywort Lycopus europaeus (Van Dijk, 1984). The impact of pre-purification on the vegetation of the seepage pools requires further research. ACKN OWLEDGE MENTS I am indebted to the following organisations for their stimulating co-operation: the Dune Water Company of The Hague, the Municipal
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Water C o m p a n y o f Amsterdam, the D u n e Water C o m p a n y of Leiden and the Hugo de Vries-Laboratory (Department of Vegetation Studies and Plant Ecology) in Amsterdam. Mrs M. J. van Hezewijk, D. de Jonge, G. C. Janze, R. Joostensz van IJsseldijk, G. van Ommering, Mrs S. Osseman, P. Otte, P. van 't Sant, H. van der Weijer and S: van der Zwan gathered m a n y of the presented data. W. T. de Groot assisted in the processing o f the data and the interpretation o f the results and T. W. M. Bakker and Mrs A. R. Kaal made critical comments on the draft,
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Van Dijk, H. W. J. & Meltzer, J. A. (1981). Hydrobiologie van natuurlijke duinmeren: een commentaar. H20, 14, 564-7. Van Oosterhoud, E., Janze, G. C., De Groot, W. T. & Van Dijk, H. W. J. (1982). Fosfaat en duininfiltratie: een experimentele benadering. H20, 15, 497-502. Wanders, E. A. J. (1982). Beheer van meeuwenkolonies in de Hollandse duinen. Duin, 5(2), 18-23. Willis, A. J. (1963). Braunton Burrows: The effects on the vegetation of the addition of mineral nutrients to the dune soils. J. Ecol., 51, 353-74.