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An insight into water stable isotope signatures in temperate catchment
Journal Pre-proofs Research papers An insight into water stable isotope signatures in temperate catchment Andis Kalvāns, Aija Dēliņa, Alise Babre, Konrāds Popovs PII: DOI: Reference:
S0022-1694(19)31177-1 https://doi.org/10.1016/j.jhydrol.2019.124442 HYDROL 124442
To appear in:
Journal of Hydrology
Received Date: Revised Date: Accepted Date:
6 September 2019 5 December 2019 6 December 2019
Please cite this article as: Kalvāns, A., Dēliņa, A., Babre, A., Popovs, K., An insight into water stable isotope signatures in temperate catchment, Journal of Hydrology (2019), doi: https://doi.org/10.1016/j.jhydrol.2019.124442
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© 2019 Published by Elsevier B.V.
Manuscript reference number: HYDROL 124442
1 Title of the manuscript An insight into water stable isotope signatures in temperate catchment
2 All authors Dr. Andis Kalvāns, University of Latvia, Faculty of Geography and Earth Sciences Dr. Aija Dēliņa, University of Latvia, Faculty of Geography and Earth Sciences Alise Babre, University of Latvia, Faculty of Geography and Earth Sciences Konrāds Popovs, University of Latvia, Faculty of Geography and Earth Sciences
3 Complete contact information of the corresponding author Andis Kalvāns Phone No.: +371 26545112 e-mail: [email protected]; [email protected] Address: Jelgavas street 1, Riga, Latvia, Lv-1004
An insight into water stable isotope signatures in temperate catchment 1
Abstract Stable isotopes are used to decipher hydrological processes in watershed research. A twoyear monthly monitoring of hydrogen and oxygen stable isotope ratios (δ2H and δ18O) in a temperate catchment in Norther Europa, Latvia was undertaken. Isotope ratios in common water types – raised bog, confined groundwater, unconfined groundwater and surface water – were measured. We found characteristic signatures of isotope ratios for each of these four water types. The average isotope ratios of different water types ranged from -80.8 to -68.3‰ for δ2H and 11.46 to -8.76‰ for δ18O, with standard deviations from 18 to 25‰ and 0.10 to 1.59‰, respectively. The isotope ratios of the stream base flow were consistent with the groundwater isoscape and seasonally enriched by evaporation. Most enriched water are found in raised bogs and large lakes. The most depleted water is found in a spring discharging phreatic groundwater in a forest site with sandy soil. The most enriched water was associated with significant shortlived precipitation events, resulting in impoundment of water on the land surface and its enrichment due to evaporation. The novelty of this study is that this enriched isotope signal is propagated throughout the hydrological system, temporarily albeit significantly shifting isotope ratios of phreatic groundwater and surface runoff. Further case studies are needed to affirm if this is a regionally significant mechanism controlling isotope ratios of surface and subsurface water. The observed difference between the average δ18O of phreatic groundwater at two locations was 0.9‰. We suggest that the differences are due to different land use and soil conditions.
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Graphical abstract
Keywords water stable isotopes, hydrogeology, catchment, runoff, evaporation, groundwater-surface water interaction
4 Introduction Ratios of water stable isotopes are a conservative tracer of water flow in the subsurface, but are modified in a predictable manner under surface conditions as reviewed by Gat (1996) and publications of the International Atomic Energy Agency (IAEA) (Gat et al., 2001; Geyh et al., 2001; Różański et al., 2001). Stable isotopes of the water molecule are often used to identify water sources and residence times in catchment studies (Farrick and Branfireun, 2015; Zhao et 3
al., 2018). Isotope tracers are used to refine conceptual models of groundwater flow and recharge (Brkić et al., 2016). In other studies, infiltration pathways of precipitation and irrigation water were investigated with stable isotopes as tracers (Ma et al., 2017); stable isotopes are used as indicators of paleoclimate (Stansell et al., 2017) or variability of groundwater recharge sources were identified linking the isotope signatures of precipitation, surface and ground water (Joshi et al., 2018). The proportion of evaporation and transpiration in a watershed, as indicated by isotope ratios, are used even as an indicator of biological net primary production (Schulte et al., 2011). Recently, groundwater isoscapes of regions like the Baltic states (Raidla et al., 2016) and the Republic of Ireland (Regan et al., 2017) have been published. Stable isotope ratios of groundwater, especially phreatic groundwater, are expected to reflect values of modern precipitation input (Clark and Fritz, 1997; Darling et al., 2003; Filippini et al., 2015; Ma et al., 2017; Rozanski, 1985) because it is the main source of water in aquifers. Groundwater recharge and variations of δ18O and δ2H are controlled by factors such as soil type, vegetation coverage and type, precipitation type and seasonality (Barbecot et al., 2018; Birkel et al., 2018; Crosbie et al., 2015; Jasechko et al., 2014; Kalvāns et al., 2018; Matiatos and Wassenaar, 2019; Raidla et al., 2016; Sánchez-Murillo and Birkel, 2016; Wassenaar et al., 2009). A study by Raidla et al. (2016) shows that stable isotope ratios in groundwater within the Baltic Artesian Basin (BAB) are close to the ratios in precipitation in the coldest seasons rather than amount of precipitation. Biases of groundwater isotope ratios towards rainy period recharge, snowmelt or event-driven recharge have been observed in Greece (Matiatos and Wassenaar, 2019). Although shallow groundwater isoscape do not directly portray annual mean weighted
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isotope rations, they can be used in forensics (Wassenaar et al., 2009) and hydrograph separations, wisely and with precaution (Regan et al., 2017). Global precipitation follows a linear relationship: δ2H = 8δ18O + 10, which is called the global meteoric water line (GMWL) (Craig, 1961). As explained by the Craig-Gordon (C-G) model (Craig and Gordon, 1965; Gonfiantini et al., 2018), isotope ratios in water remaining in liquid and vapour phases deviate from GMWL during evaporation; the resulting relationship between δ2H and δ18O is sometimes called the evaporation line. The slope of the evaporation line depends on conditions (temperature, relative humidity, ambient vapor isotopic composition, mixing at the water-air interface, and the thermodynamic activity (salinity) of water) (Gonfiantini et al., 2018) during evaporation and is less steep (or flatter) than the GMWL. In this study we measured the stable isotope ratios of different, interlinked types of surface and groundwater in a hemiboreal lowland catchment. Every month for two years, we observed isotopic ratios in a large lake (Lake Burtnieks), groundwater and a raised bog as well as in the River Salaca drainage which emerges from the lake and its tributaries. Unexpectedly, we found strong heavy-isotope enrichment that was propagated through the hydrological cycle during periods of snowmelt and excessive precipitation that were associated with water impoundment on land surface.
5 Site description 5.1 Physical geography This study was conducted on the scale of the watershed, an upstream section of the River Salaca in the north-eastern part of Latvia near the Gulf of Riga in the Baltic Sea (Fig. 1). Its drainage basin is situated in the lowlands of northern Latvia and southern Estonia. The Salaca 5
headwater is the shallow Lake Burtnieks. The lake is the main source of river flow and moderates seasonal discharge parameters. Input from tributaries is estimated at about 14% (Table 1), however the groundwater contribution is not known.
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Fig. 1. (a) overview and (b) detailed map of the study location; (c) close up orthophoto map of the Ramata groundwater sampling site; see the sampling site description in Supplementary material 1.
A 43-km section of the River Salaca starting at Lake Burtnieks was considered. In this section, the river flows through a valley carved into poorly consolidated Devonian sandstones capped by clay-rich glacial till and outwash sediments. Sandstone cliffs are punctuated by springs and suffusion caves associated with springs. The valley floor is as much as 20 m below the surrounding landscape.
5.2 Climate and hydrology According to the Köppen-Geiger classification, he study site has a warm summer, humid continental (Dfb, hemiboreal) climate (Klocking et al., 2006). The average annual air temperature is +5.9 °C; the warmest month is July with an average temperature of +17 °C, while the coldest months are January and February with an average temperature between -4° and -5 °C (LVĢMC, 2017). The average annual precipitation in the study region varies from 700 to 850 mm (Briede and Koreļska, 2018). The average monthly precipitation is highest in July and August (respectively 77 and 76 mm/month) and lowest in February, March and April (around 33 mm/month) (LVĢMC, 2017). It has been estimated that 62% of the total amount and 80% of summer precipitation is lost to evapotranspiration (Briede and Rodinovs, 1993). Annual fluctuations of the groundwater table indicating periods of recharge were characterised by an M-shaped curve. One maximum occurred in April due to infiltration of, and another maximum began in September and culminated in December (Tolstovs et al., 1986) reflecting increased precipitation and low evapotranspiration in autumn. Precipitation water is 7
evapotranspirated in summer, while freezing conditions in winter preclude infiltration (Tolstovs et al., 1986). However, due to milder winters in recent years, the two groundwater table maxima tend to merge (Lauva et al., 2012). In the catchment of the Salaca River the runoff is primarily generated by precipitation, that is classified as marine conditions by Kriauciuniene et al. (2012). The long-term average specific drainage of the middle reaches of the Salaca River is 8 L/s km2 annually and 2–3.5 L/s km2 in summer. The flow velocity in summer is 0.1 to 0.3 m/s down to Mazsalaca and 0.3 to 0.4 m/s downstream of Mazsalaca (Briede and Rodinovs, 1993). It takes one or two days for water to travel the 43 km from the lake's outlet to the lowermost observation site. Macrophyte overgrowth on the river in summertime exceeds 30% (Grīnberga and Spriņģe, 2008), diminishing the flow velocity. The catchment area of the Salaca River tributaries, Iģe and Ramata, are 212 and 190 km2 respectively, while the catchment of the Salaca itself from the outlet to the entry of the Iģe is 203 km2. The land surface is mostly covered by agriculture, forests and raised bogs (Table 1); urban/industrial areas comprise less than 1%. The extent of the wetlands might be underestimated in the CORINE database (European Environment Agency, 2016) as some sources report up to 15% bog cover (Briede and Rodinovs, 1993). The soil is composed of sands, loam (glacial till) or peat. The catchment of the small River Piģele is almost entirely (83%) covered by raised bogs (Klavins et al., 2012).
Table 1. Catchment area. Average runoff (LVĢMC, 2015a) and land cover classification (European Environment Agency, 2016).
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Sub-catchment Area (km2) Average runoff (m3/s) Forest Agriculture Wetland Water Lake Burtnieks
2 119
54 %
39 %
3%
3%
River Ramata
231
1.71
69 %
25 %
5%
1%
River Iģe
212
2.21
58 %
37 %
5%
0%
River Salaca
203*
27.16
53* %
37* %
8* %
1* %
* without tributaries Ramata and Iģe and Lake Burtnieks
Lake Burtnieks is on average 2.4 m deep. Its surface area is 40 km2 and its catchment occupies 2215 km2 (Apsīte et al., 2012). Several rivers feed into the lake (Rūja, Seda, Briede and a few smaller ones), while the River Salaca is its sole outlet. Water turnover in Lake Burtnieks occurs six to seven times per year, with a residence times of two to three weeks in spring and three months in summer (Apsīte et al., 2012). The total dissolved solids (TDS) in the lake range between 370 to 400 mg/l in winter and 280 to 320 mg/l in summer (Rodinovs and Kļaviņš, 1993). A small hydropower plant (Rauskas HPP) with a water reservoir is located on the Ramata River (Fig. 1). During low-flow periods, episodic operation of the hydropower plant manifests as fluctuations in the water table and electrical conductivity (EC) along the river.
5.3 Geology The study region is in the East European Plain, well within the extent of the last Scandinavian glaciation (Kalm, 2012). The Salaca River catchment is characterised by undulating moraine and the elevation ranges from 33 m to 125 m above sea level (a.s.l.). The Middle Devonian sediments are represented by the Burtnieks and Arukila formations belonging 9
to Givetina and Eifelian chronostratigraphic stages, and composed of sandstone, sand, siltstone and clay. The Devonian sediments are exposed on the slopes of the River Salaca valley. These rest on top of the Narva regional aquiclude (Eifelian stage) mostly composed of marls and other carbonate and clay-rich sediments and interlayers of gypsum (Lukševičs et al., 2012). The thickness of the Arukila and Burtnieks formations are around 70 m and up to 50 m, respectively. They are overlain by Quaternary glacial till and, in places, glacial outwash sediments – gravel and sand (Lukševičs et al., 2012). The Quaternary cover usually varies between 10 and 20 m thick, rarely reaching 40 m in hilly landscapes (Popovs et al., 2015).
5.4 Background precipitation and groundwater isotope ratios The precipitation isotope ratios data are available at the IAEA/GNIP (Global Network of Isotopes in Precipitation) data base. The three closest stations are: Tartu (110 km), Riga (120 km) and Vilsandi (190 km) (IAEA/WMO, 2019). The notion of d-excess (deuterium excess) is a convenient way to illustrate the deviation of water isotope observation from the GMWL, calculated as d-excess = δ2H - 8*δ18O (Craig, 1961), indicating the deviation of the δ2H and δ18O. The weighted d-excess for local precipitation at the Riga station, 120 km to the south of the study site was 11.2‰ (Babre et al., 2016). In Tartu, a continental location 110 km from the study site, it was 8.63‰, and in Vilsandi, a marine location 190 km from the study site, it was 9.10‰ (IAEA/WMO, 2019). The observation period was from 1981 to 1984 in Riga and from 2013 to 2015 in Tartu and Vilsandi. The least squares fit (LSF) regression lines between two isotope ratios are: δ2H = 7.45 * δ18O + 5.67 (Riga), δ2H = 7.94 * δ18O + 7.75 (Tartu) and δ2H = 8.23 * δ18O + 10.7 (Vilsandi). 10
The background isoscape for δ18O in groundwater was interpolated from direct observations by Raidla et al. (2016). It vary from -10.8 to -11.5‰. The average of 10 direct δ18O groundwater observations (Babre et al., 2016; Raidla et al., 2016) in the study area is -11.34‰ ± 0.19‰ standard deviations (SD). Direct observations are consistent with interpolated values of the isoscape. The interpolated groundwater isoscape is spatially continuous; therefore, in this study it is used as a reference instead of individual direct observations. The mean of 7 direct δ2H groundwater observations (Babre et al., 2016; Raidla et al., 2016) is -78.9% ± 2.2% SD. The linear regression line between observed δ18O and δ2H has a slope of 12 ± 3 SD and an intercept of 55 ± 36 SD; note the high uncertainty.
6 Materials and methods 6.1 Meteorological and hydrological data The meteorological data – temperature and precipitation – was obtained from meteorological stations Rūjiena, Ainaži and Priekuļi (Fig. 1). These stations are 16 to 33, 30 to 50 and 55 to 75 km, respectively, from the study region; their elevations are 6.28, 67.71 and 122.07 m a.s.l., respectively. River Salaca water levels and discharge were measured at two national observation stations (Fig. 1(b)). Meteorological and hydrological observation stations are operated by the national meteorological agency "Latvian Environment, Geology and Meteorology Centre."
6.2 Sampling strategy and methodology The sampling strategy was to observe common water types in the study region using precipitation data and discharge data from Lake Burtnieks, tributaries to the River Salaca, raised
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bog and groundwater (Fig. 1 and 2). Detailed descriptions are given in Supplementary material – 1.
Fig. 2. Conceptual model of runoff in the upper Salaca catchment: 1 – precipitation; 2 – drainage from raised bogs (LU1, PP1, PV1); 3 – springs discharging in the River Salaca valley (GA1); 4 –confined groundwater presumed to discharge directly into the River Salaca (GU1); 5 – phreatic groundwater draining from till terrain with fine grained cultivated soils (RU1, RU2, RU3); 6 – tributaries Ramata and Iģe (RV1, IV1); 7 – River Salaca channel (SV1, SV2, SV3) mostly originating from Lake Burtnieks.
Samples were collected monthly for 25 consecutive months starting in August 2015, generally at the last days of each month. For two months – July and November 2016 – samples were collected every two days. The first period of intensive sampling corresponds to midsummer conditions with the strongest modulation in surface water isotope values likely due to evaporation. While, the second period was during the transition from summer to winter conditions, and shifting isotopic values in surface waters were expected. EC and pH were measured in the field using a WTW multimeter Multiline 3420 with a TetraCone 925 probe for EC and temperature, and a SenTix 940 probe for pH and temperature at 12
each sampling location. Precipitation was measured when collecting the isotope samples. Samples, including duplicates, were collected in 25 ml high density polyethylene (HDPE) bottles without filtration. River water was sampled with buckets directly from the stream surface and immediately transferred to a sample bottle. Groundwater was sampled from temporary monitoring wells with a 1-m long filter using either submersible or peristaltic pumps. Before sampling, all water was removed from the well so that fresh water flowing into the well could be sampled or pumped until EC and pH values stabilised.
6.3 Laboratory procedures Stable isotope ratios of hydrogen and oxygen in water were analysed in the Laboratory of Environmental Dating at the University of Latvia (Faculty of Geography and Earth Sciences). Isotope ratios are expressed in standard δ-notation relative to the Vienna Standard Mean Ocean Water (VSMOW; Craig, 1961). Both isotope ratios of hydrogen and oxygen were measured using the cavity ring-down laser spectroscopy method (Brand et al., 2009) with a Picarro L2120-i Isotopic Water Analyzer. Reproducibility of stable isotope measurements was less than ±0.1‰ for δ18O and ±1‰ for δ2H. To assure quality of water sampling and processing in the laboratory, internationally-accepted procedures elaborated by the IAEA (Aggarwal et al., 2007) were followed. The laboratory has successfully participated in world-wide open proficiency tests on the determination of stable isotopes in water organised by the IAEA in 2016 (Wassenaar et al., 2018).
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7 Results 7.1
Hydrometeorological conditions The observed precipitation rarely exceeded 50mm/week and was evenly distributed
throughout the year (Fig. 3), approximately corresponding with long-term average observations. One exception occurred in August 2016 when the monthly precipitation at the Rūjiena meteorological station was 233 mm.
Fig. 3. (a) weekly precipitation in Ainaži, Priekuļi and Rūjiena meteorological stations; (b) relative air humidity in Rūjiena meteorological station and (c); air temperature in Rūjiena
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meteorological station and water temperature in the River Salaca (observation site SV3) from August 2015 to September 2017.
During the observation period, summer and early autumn was characterised by low levels of both surface water (Fig. 4) and groundwater tables (Fig. 5); while in winter and spring, water tables were high. At the Mazsalaca hydrological station, the ice cover was present in January 2016 and January to mid-February 2017, reflecting relatively mild winters. It is assumed that a stable water temperature close to 0°C indicates ice cover at SV1 – the headwater of the River Salaca (Fig. 4). According to this indicator, the ice cover in winter 2015/2016 was less than three months, and in 2016/2017, it was around four months. The average duration of ice cover on Lake Burtnieks from 1988 to 2001 was 116 days (Apsīte et al., 2012), almost four months.
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Fig. 4. River Salaca: (a) observed relative water table, (b) electrical conductivity, (c) temperature; (d) δ18O and € d-excess values in the River Salaca at the outlet of Lake Burtnieks (SV1) and downstream locations (Mazsalaca, SV2, SV3 and Lagaste)
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Fig. 5. Tributaries: (a) observed relative water table, (b) electrical conductivity, (c) temperature; (d) δ18O and (e) d-excess values for the small rivers Iģe (IV1) and Ramata (RV1).
7.2 Isotope ratios In total isotope rations were measured in 554 samples. However, several samples were discarded after quality check. Initially, two precipitation traps were set up in accordance with IAEA/GNIP (2014) guidelines; but evaporation, partial shading by vegetation, blocking of tubing by ice and vegetative litter, and a change in methodology compromised the sample quality. Therefore, all the precipitation samples were excluded from the analysis in this paper. Finally, results from detailed observations (two month of measurements every other day) are not examined in this paper as well. 17
Fig. 6. (a, b) observed range of isotope ratios and (c) electrical conductivity; the grey zone in the δ18O plot represents the groundwater isoscape and its uncertainty (Raidla et al., 2016) across the entire catchment.
All samples considered after quality control were within the range of -6.65 to -14.4 δ18O‰ and -60.8 to -100.9 for δ2H‰; d-excess varied between -11.2 and 21.9‰ (Fig. 6). Groundwater samples (GU1, RU1, RU2, RU3) including the spring (GA1) showed more stable results for all parameters throughout the observation period and have the most depleted average values (standard deviations of 0.1 for oxygen-18 and 0.8 for deuterium). Bog samples (PP, PV1, LU1) showed significantly enriched isotope ratios compared to other sampling sites and had the 18
lowest mean deuterium d-excess (1.8‰). Isotope ratios of the samples reflect the local evaporation line, while phreatic groundwater (GA1, RU1, RU2, RU3) and groundwater (GU1) samples mostly coincide with the GMWL (Fig. 7). The lowest mean EC values are from raised bog, indicating atmospheric recharge, while the highest values were obtained from shallow groundwater samples (Fig. 6). The broadest range of EC was for surface water samples (Fig. 6).
Fig. 7. Dual isotope plots of observed δ18O and δ2H values; grey vertical zone is the groundwater isoscape from (Raidla et al., 2016), long diagonal line is GMWL and the short diagonal line with uncertainty is the linear regression from the data points.
Essentially all water types (Fig. 4., 5., 8., 9) except groundwater (Fig. 10) have seasonal fluctuations in isotope ratios. Isotope seasonality can be smooth and pronounced like in the River
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Salaca and its tributaries (Fig. 4 and 5) or event-driven as in phreatic groundwater and raised bogs (Fig. 8 and 9).
Fig. 8. Phreatic groundwater: (a) observed relative water table, (b) temperature, (c) δ18O, (d) d-excess values and (e) electrical conductivity at the spring (GA1), groundwater monitoring wells (RU1 and RU2) and an agricultural drain outlet (RU3).
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Fig. 9. Raised bog: (a) observed relative water table, (b) temperature, (c) δ18O, (d) dexcess values and (e) electrical conductivity at the shallow monitoring well in raised bog (LU1) and River Pigele within raised bog (PP) and outside of it at a downstream location (PV1).
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Fig. 10. Groundwater: (a) observed (b) δ18O, (c) d-excess values and electrical conductivity (c) at the groundwater monitoring well (GU1).
8 Discussion 8.1 The wet events The most extreme average monthly isotope ratios – January to February 2016, August to October 2016 and February to May 2017 (Fig. 11) – can be linked to meteorological conditions.
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Fig. 11. Average oxygen isotope ratios in different water types. Three events of enhanced groundwater recharge are highlighted: 1. – thaw (melting snow) in January 2016; 2. – strong precipitation in August 2016 followed by relatively dry and warm September; 3. – thaw (snow melt) in February 2017 followed by a warm and dry March and April.
8.2 Winter thaw January 2016 At the end of January 2016, after several weeks of freezing temperatures, a thaw event occurred. In the tile drainage (RU3), strongly depleted water with low EC was observed (Fig. 8), indicating preferential infiltration of snowmelt. To a lesser extent, this depletion signal was evident in tributaries and, with a one-month delay in the raised bog and Salaca River (Fig. 4, 5 and 9).
8.3 Extreme precipitation in August 2016 In August 2016, exceptionally high precipitation was observed (monthly total of 233 mm and 150 mm in a single event on August 23 at Rūjiena meteorological station, Fig. 3); that is 23
approximately one-quarter of the yearly mean precipitation (Briede and Koreļska, 2018). September 2016 had moderate precipitation (28.8 mm, Rūjiena meteorological station), the average air temperature was 13°C and the average relative air humidity was 84% (Fig. 3). The mean phreatic groundwater δ18O and d-excess in August 2016 were -9.7‰ and 3.5‰, respectively (Fig. 11), indicating enrichment by evaporation. Following this event, enriched δ18O was observed in the River Salaca and raised bog as well. A spike of enriched water at the end of August 2016 was found in the River Iģe as well, but not in the River Ramata (Fig. 5). Enriched water averaging δ18O = -10.7‰ (Fig. 8) was observed in Govs Ala spring (GA1) up to three months after this event, while the two-year mean was -11.4‰ with a standard deviation of 0.33. The spring discharges from a sandy phreatic aquifer as evident from the outcrop of poorly consolidated sandstones covered by glacial outwash sediments (State Geological Survey, n.d.). As determined from the topographical map, the thickness of unsaturated sediments above the spring discharge site is around 16 m. The observed excursion of isotope ratios can be explained by preferential infiltration of an unusually large volume of precipitation in this event (Fig. 3).
8.4 Thaw in February 2017 Phreatic groundwater (RU1, RU2, RU3) and Lake Burtnieks (SV1) samples collected at the end of February 2017 had noticeably depleted δ18O values, while water in the tributaries and downstream sampling sites on the River Salaca (SV2 and SV3) had enriched values. Enriched δ18O values were already observed in phreatic groundwater and surface runoff (Fig. 4., 5., 8., 9.) at the end of March 2017. The δ18O enrichment is associated with decreasing d-excess values indicating modification by evaporation. Enriched water was observed in phreatic groundwater samples up to three months after the event (Fig. 11). The same pattern was observed in the raised bog, but with a one-month delay (Fig. 11). 24
It appears that in February 2017, due to the combination of snowmelt and precipitation, a large volume of depleted water appeared in the catchment. Some of it was drained as surface runoff, some entered the groundwater system and some remained on the frozen soil surface. The average air temperature in March 2017 was 1.4 °C and the average relative air humidity was 82% (Fig. 3), thus facilitating evaporation. It is likely that water stagnating on the soil surface attained increasingly enriched isotope values due to evaporation, and as frozen soil was thawing, infiltrated the groundwater and drained into streams.
8.5 Generalisation and implications We suggest that at these three episodes – the impoundment of the water on the soil surface and subsequent infiltration of water enriched by evaporation (Fig. 12) – are the dominant mechanisms producing the observed excursions in isotope ratios (Fig. 11). We suggest that the intensity-distribution of precipitation has significant control on surface and ground water isotope ratios in terrestrial conditions. Enriched water-derived isotope ratios and decrease of d-excess in paleorecords may be due to increased precipitation and its intensity rather than a shift towards more arid conditions. Isotope seasonality is controlled by the complex interplay between precipitation, evaporation, transpiration and hydrological conditions. Impoundment of precipitation water on land surface can occur when rapid snowmelt takes place over frozen ground, when soil permeability is insufficient to drain incoming precipitation or when the groundwater table reaches the soil surface. During events in spring 2016 and 2017 rapid infiltration of precipitation and snowmelt water likely was precluded by froze soil. The event in August–September 2016 likely is combination precipitation intensity exceeding the soil infiltration capacity and groundwater table reaching soil surface at places.
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Described events are associated with decreasing EC in the River Salaca and its tributaries (Fig. 4 and 5), indicating input of unaltered precipitation water. The magnitude of EC fluctuations is in line with those previously observed (Rodinovs and Kļaviņš, 1993).
Fig. 12. Formation of isotopically-enriched groundwater due to water accumulation in puddles on frozen soil, if average d-excess in precipitation is 10‰.
Alternatively, evolution of isotope ratios can be explained by mobilisation of enriched water stored in topsoil (Sprenger et al., 2016) due to sublimation of the snowpack (Stichler et al., 2001) and infiltration of melt water, or by evaporation of rain or snow trapped by the canopy before reaching the soil and groundwater (Vystavna et al., 2018), or precipitation water may already have a particularly-low d-excess. In addition, enhanced surface runoff can trigger mobilisation of isotopically-enriched water stored in bogs and ponds. Potentially, this can have 26
noticeable effects on isotopic signatures in rivers, as the estimated wetland area in this catchment is between 5% (European Environment Agency, 2016) and 15% (Briede and Rodinovs, 1993). Meteorological conditions that can lead to water impoundment on land surface and cause significant enrichment before entering the subterranean and surface runoff system seem to occur frequently. In Estonia, just north of the study area, during the warm season, there were up to four days of extreme precipitation (exceeding the 99th percentile, R99p, > 20 mm/day). Due to climate change, the number of such extreme events is expected to increase (Päädam and Post, 2011), thus changes in precipitation intensity distribution alone can affect the isotope ratios of groundwater. Kriauciuniene et al. (2012) indicated that the runoff of Salaca River is primarily generated by precipitation implying surface runoff or short-cycle groundwater runoff. Given the predominantly lowland plain type of terrain, that does not favour swift surface runoff, the River Salaca catchment has favourable conditions for isotopic enrichment of precipitation water by the mechanism proposed. We can speculate that in other regions of similar dominant mechanism of river runoff generation mechanisms identified by (Kriauciuniene et al., 2012) may indicate areas where the proposed mechanism is an important factor contribution to isotope ratios of surface and groundwater. Additional studies are clearly needed to assess whether evaporative enrichment of water temporarily residing on the land surface is a significant mechanism contributing to the groundwater isotope ratios. We suggest that water isotope ratios δ18O and δ2H are valuable indicators for groundwater monitoring in line with the EU Water framework directive (The European Parliament and the Council of the European Union, 2000) to better understand evolving changes in recharge conditions and surface-groundwater interactions. Water stable isotopes already have 27
been used as indicators of groundwater recharge sources (Joshi et al., 2018) or to explain variations of groundwater quality (Sarkar et al., 2017). Free-flowing springs like Govs Ala spring (GA1) are often included in groundwater monitoring programmes (LVĢMC, 2015b) as a costeffective means of obtaining groundwater samples without the need for heavy pumping equipment. Episodic discharge of preferential infiltration as indicated by a spike in isotope ratios in August to October 2016 (sampling site GA1, Fig. 8) can complicate or even disable meaningful interpretation of results. Water quality parameters measured during such episodes would be difficult if not impossible to interpret and to generalise over a wider region.
9
Conclusions We found characteristic signatures of isotope ratios for each type of water studied: water
in raised bogs, confined groundwater, phreatic groundwater in terrains with fine-grained cultivated soils and forested sandy soils, and river and lake water. On average the most enriched water was found in raised bog (δ18O = -8.84 ‰) and river emanating from Lake Burtnieks (δ18O = -9.18‰). The most depleted water was found in a spring discharging phreatic groundwater at a forest site with sandy soil (δ18O = -11.4‰). We suggest that evaporation of water impounded on the soil surface is an important mechanism leading to isotopic enrichment of surface and subsurface water. Further case studies are needed to test if this is a regionally significant mechanism contributing to the surface and groundwater isotope ratios.
28
10 Acknowledgements Funding: This work was supported by the Latvian National Research Programme EVIDEnT (No. 10-4/VPP-2/19), subproject 5.2 and grant No. ZD2016/AZ03 "Climate change and sustainable utilization of natural resources” at the University of Latvia. The data set is available as Data in Breif publication “Water stable isotope data set in temperate, lowland catchment, two years of monthly observations”.
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Highlights 1. Monthly water isotope observations for two years at 13 sampling points in a temperate catchment 2. Distinct water isotope signatures in raised bog, groundwater and surface water 3. Surface water isotope seasonality due to evapotranspiration rather than precipitation 4. Isotope enrichment of water impounded on soil surface is propagated to the groundwater and streams
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S0022-1694(19)31177-1 https://doi.org/10.1016/j.jhydrol.2019.124442 HYDROL 124442
To appear in:
Journal of Hydrology
Received Date: Revised Date: Accepted Date:
6 September 2019 5 December 2019 6 December 2019
Please cite this article as: Kalvāns, A., Dēliņa, A., Babre, A., Popovs, K., An insight into water stable isotope signatures in temperate catchment, Journal of Hydrology (2019), doi: https://doi.org/10.1016/j.jhydrol.2019.124442
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Manuscript reference number: HYDROL 124442
1 Title of the manuscript An insight into water stable isotope signatures in temperate catchment
2 All authors Dr. Andis Kalvāns, University of Latvia, Faculty of Geography and Earth Sciences Dr. Aija Dēliņa, University of Latvia, Faculty of Geography and Earth Sciences Alise Babre, University of Latvia, Faculty of Geography and Earth Sciences Konrāds Popovs, University of Latvia, Faculty of Geography and Earth Sciences
3 Complete contact information of the corresponding author Andis Kalvāns Phone No.: +371 26545112 e-mail: [email protected]; [email protected] Address: Jelgavas street 1, Riga, Latvia, Lv-1004
An insight into water stable isotope signatures in temperate catchment 1
Abstract Stable isotopes are used to decipher hydrological processes in watershed research. A twoyear monthly monitoring of hydrogen and oxygen stable isotope ratios (δ2H and δ18O) in a temperate catchment in Norther Europa, Latvia was undertaken. Isotope ratios in common water types – raised bog, confined groundwater, unconfined groundwater and surface water – were measured. We found characteristic signatures of isotope ratios for each of these four water types. The average isotope ratios of different water types ranged from -80.8 to -68.3‰ for δ2H and 11.46 to -8.76‰ for δ18O, with standard deviations from 18 to 25‰ and 0.10 to 1.59‰, respectively. The isotope ratios of the stream base flow were consistent with the groundwater isoscape and seasonally enriched by evaporation. Most enriched water are found in raised bogs and large lakes. The most depleted water is found in a spring discharging phreatic groundwater in a forest site with sandy soil. The most enriched water was associated with significant shortlived precipitation events, resulting in impoundment of water on the land surface and its enrichment due to evaporation. The novelty of this study is that this enriched isotope signal is propagated throughout the hydrological system, temporarily albeit significantly shifting isotope ratios of phreatic groundwater and surface runoff. Further case studies are needed to affirm if this is a regionally significant mechanism controlling isotope ratios of surface and subsurface water. The observed difference between the average δ18O of phreatic groundwater at two locations was 0.9‰. We suggest that the differences are due to different land use and soil conditions.
2
Graphical abstract
Keywords water stable isotopes, hydrogeology, catchment, runoff, evaporation, groundwater-surface water interaction
4 Introduction Ratios of water stable isotopes are a conservative tracer of water flow in the subsurface, but are modified in a predictable manner under surface conditions as reviewed by Gat (1996) and publications of the International Atomic Energy Agency (IAEA) (Gat et al., 2001; Geyh et al., 2001; Różański et al., 2001). Stable isotopes of the water molecule are often used to identify water sources and residence times in catchment studies (Farrick and Branfireun, 2015; Zhao et 3
al., 2018). Isotope tracers are used to refine conceptual models of groundwater flow and recharge (Brkić et al., 2016). In other studies, infiltration pathways of precipitation and irrigation water were investigated with stable isotopes as tracers (Ma et al., 2017); stable isotopes are used as indicators of paleoclimate (Stansell et al., 2017) or variability of groundwater recharge sources were identified linking the isotope signatures of precipitation, surface and ground water (Joshi et al., 2018). The proportion of evaporation and transpiration in a watershed, as indicated by isotope ratios, are used even as an indicator of biological net primary production (Schulte et al., 2011). Recently, groundwater isoscapes of regions like the Baltic states (Raidla et al., 2016) and the Republic of Ireland (Regan et al., 2017) have been published. Stable isotope ratios of groundwater, especially phreatic groundwater, are expected to reflect values of modern precipitation input (Clark and Fritz, 1997; Darling et al., 2003; Filippini et al., 2015; Ma et al., 2017; Rozanski, 1985) because it is the main source of water in aquifers. Groundwater recharge and variations of δ18O and δ2H are controlled by factors such as soil type, vegetation coverage and type, precipitation type and seasonality (Barbecot et al., 2018; Birkel et al., 2018; Crosbie et al., 2015; Jasechko et al., 2014; Kalvāns et al., 2018; Matiatos and Wassenaar, 2019; Raidla et al., 2016; Sánchez-Murillo and Birkel, 2016; Wassenaar et al., 2009). A study by Raidla et al. (2016) shows that stable isotope ratios in groundwater within the Baltic Artesian Basin (BAB) are close to the ratios in precipitation in the coldest seasons rather than amount of precipitation. Biases of groundwater isotope ratios towards rainy period recharge, snowmelt or event-driven recharge have been observed in Greece (Matiatos and Wassenaar, 2019). Although shallow groundwater isoscape do not directly portray annual mean weighted
4
isotope rations, they can be used in forensics (Wassenaar et al., 2009) and hydrograph separations, wisely and with precaution (Regan et al., 2017). Global precipitation follows a linear relationship: δ2H = 8δ18O + 10, which is called the global meteoric water line (GMWL) (Craig, 1961). As explained by the Craig-Gordon (C-G) model (Craig and Gordon, 1965; Gonfiantini et al., 2018), isotope ratios in water remaining in liquid and vapour phases deviate from GMWL during evaporation; the resulting relationship between δ2H and δ18O is sometimes called the evaporation line. The slope of the evaporation line depends on conditions (temperature, relative humidity, ambient vapor isotopic composition, mixing at the water-air interface, and the thermodynamic activity (salinity) of water) (Gonfiantini et al., 2018) during evaporation and is less steep (or flatter) than the GMWL. In this study we measured the stable isotope ratios of different, interlinked types of surface and groundwater in a hemiboreal lowland catchment. Every month for two years, we observed isotopic ratios in a large lake (Lake Burtnieks), groundwater and a raised bog as well as in the River Salaca drainage which emerges from the lake and its tributaries. Unexpectedly, we found strong heavy-isotope enrichment that was propagated through the hydrological cycle during periods of snowmelt and excessive precipitation that were associated with water impoundment on land surface.
5 Site description 5.1 Physical geography This study was conducted on the scale of the watershed, an upstream section of the River Salaca in the north-eastern part of Latvia near the Gulf of Riga in the Baltic Sea (Fig. 1). Its drainage basin is situated in the lowlands of northern Latvia and southern Estonia. The Salaca 5
headwater is the shallow Lake Burtnieks. The lake is the main source of river flow and moderates seasonal discharge parameters. Input from tributaries is estimated at about 14% (Table 1), however the groundwater contribution is not known.
6
Fig. 1. (a) overview and (b) detailed map of the study location; (c) close up orthophoto map of the Ramata groundwater sampling site; see the sampling site description in Supplementary material 1.
A 43-km section of the River Salaca starting at Lake Burtnieks was considered. In this section, the river flows through a valley carved into poorly consolidated Devonian sandstones capped by clay-rich glacial till and outwash sediments. Sandstone cliffs are punctuated by springs and suffusion caves associated with springs. The valley floor is as much as 20 m below the surrounding landscape.
5.2 Climate and hydrology According to the Köppen-Geiger classification, he study site has a warm summer, humid continental (Dfb, hemiboreal) climate (Klocking et al., 2006). The average annual air temperature is +5.9 °C; the warmest month is July with an average temperature of +17 °C, while the coldest months are January and February with an average temperature between -4° and -5 °C (LVĢMC, 2017). The average annual precipitation in the study region varies from 700 to 850 mm (Briede and Koreļska, 2018). The average monthly precipitation is highest in July and August (respectively 77 and 76 mm/month) and lowest in February, March and April (around 33 mm/month) (LVĢMC, 2017). It has been estimated that 62% of the total amount and 80% of summer precipitation is lost to evapotranspiration (Briede and Rodinovs, 1993). Annual fluctuations of the groundwater table indicating periods of recharge were characterised by an M-shaped curve. One maximum occurred in April due to infiltration of, and another maximum began in September and culminated in December (Tolstovs et al., 1986) reflecting increased precipitation and low evapotranspiration in autumn. Precipitation water is 7
evapotranspirated in summer, while freezing conditions in winter preclude infiltration (Tolstovs et al., 1986). However, due to milder winters in recent years, the two groundwater table maxima tend to merge (Lauva et al., 2012). In the catchment of the Salaca River the runoff is primarily generated by precipitation, that is classified as marine conditions by Kriauciuniene et al. (2012). The long-term average specific drainage of the middle reaches of the Salaca River is 8 L/s km2 annually and 2–3.5 L/s km2 in summer. The flow velocity in summer is 0.1 to 0.3 m/s down to Mazsalaca and 0.3 to 0.4 m/s downstream of Mazsalaca (Briede and Rodinovs, 1993). It takes one or two days for water to travel the 43 km from the lake's outlet to the lowermost observation site. Macrophyte overgrowth on the river in summertime exceeds 30% (Grīnberga and Spriņģe, 2008), diminishing the flow velocity. The catchment area of the Salaca River tributaries, Iģe and Ramata, are 212 and 190 km2 respectively, while the catchment of the Salaca itself from the outlet to the entry of the Iģe is 203 km2. The land surface is mostly covered by agriculture, forests and raised bogs (Table 1); urban/industrial areas comprise less than 1%. The extent of the wetlands might be underestimated in the CORINE database (European Environment Agency, 2016) as some sources report up to 15% bog cover (Briede and Rodinovs, 1993). The soil is composed of sands, loam (glacial till) or peat. The catchment of the small River Piģele is almost entirely (83%) covered by raised bogs (Klavins et al., 2012).
Table 1. Catchment area. Average runoff (LVĢMC, 2015a) and land cover classification (European Environment Agency, 2016).
8
Sub-catchment Area (km2) Average runoff (m3/s) Forest Agriculture Wetland Water Lake Burtnieks
2 119
54 %
39 %
3%
3%
River Ramata
231
1.71
69 %
25 %
5%
1%
River Iģe
212
2.21
58 %
37 %
5%
0%
River Salaca
203*
27.16
53* %
37* %
8* %
1* %
* without tributaries Ramata and Iģe and Lake Burtnieks
Lake Burtnieks is on average 2.4 m deep. Its surface area is 40 km2 and its catchment occupies 2215 km2 (Apsīte et al., 2012). Several rivers feed into the lake (Rūja, Seda, Briede and a few smaller ones), while the River Salaca is its sole outlet. Water turnover in Lake Burtnieks occurs six to seven times per year, with a residence times of two to three weeks in spring and three months in summer (Apsīte et al., 2012). The total dissolved solids (TDS) in the lake range between 370 to 400 mg/l in winter and 280 to 320 mg/l in summer (Rodinovs and Kļaviņš, 1993). A small hydropower plant (Rauskas HPP) with a water reservoir is located on the Ramata River (Fig. 1). During low-flow periods, episodic operation of the hydropower plant manifests as fluctuations in the water table and electrical conductivity (EC) along the river.
5.3 Geology The study region is in the East European Plain, well within the extent of the last Scandinavian glaciation (Kalm, 2012). The Salaca River catchment is characterised by undulating moraine and the elevation ranges from 33 m to 125 m above sea level (a.s.l.). The Middle Devonian sediments are represented by the Burtnieks and Arukila formations belonging 9
to Givetina and Eifelian chronostratigraphic stages, and composed of sandstone, sand, siltstone and clay. The Devonian sediments are exposed on the slopes of the River Salaca valley. These rest on top of the Narva regional aquiclude (Eifelian stage) mostly composed of marls and other carbonate and clay-rich sediments and interlayers of gypsum (Lukševičs et al., 2012). The thickness of the Arukila and Burtnieks formations are around 70 m and up to 50 m, respectively. They are overlain by Quaternary glacial till and, in places, glacial outwash sediments – gravel and sand (Lukševičs et al., 2012). The Quaternary cover usually varies between 10 and 20 m thick, rarely reaching 40 m in hilly landscapes (Popovs et al., 2015).
5.4 Background precipitation and groundwater isotope ratios The precipitation isotope ratios data are available at the IAEA/GNIP (Global Network of Isotopes in Precipitation) data base. The three closest stations are: Tartu (110 km), Riga (120 km) and Vilsandi (190 km) (IAEA/WMO, 2019). The notion of d-excess (deuterium excess) is a convenient way to illustrate the deviation of water isotope observation from the GMWL, calculated as d-excess = δ2H - 8*δ18O (Craig, 1961), indicating the deviation of the δ2H and δ18O. The weighted d-excess for local precipitation at the Riga station, 120 km to the south of the study site was 11.2‰ (Babre et al., 2016). In Tartu, a continental location 110 km from the study site, it was 8.63‰, and in Vilsandi, a marine location 190 km from the study site, it was 9.10‰ (IAEA/WMO, 2019). The observation period was from 1981 to 1984 in Riga and from 2013 to 2015 in Tartu and Vilsandi. The least squares fit (LSF) regression lines between two isotope ratios are: δ2H = 7.45 * δ18O + 5.67 (Riga), δ2H = 7.94 * δ18O + 7.75 (Tartu) and δ2H = 8.23 * δ18O + 10.7 (Vilsandi). 10
The background isoscape for δ18O in groundwater was interpolated from direct observations by Raidla et al. (2016). It vary from -10.8 to -11.5‰. The average of 10 direct δ18O groundwater observations (Babre et al., 2016; Raidla et al., 2016) in the study area is -11.34‰ ± 0.19‰ standard deviations (SD). Direct observations are consistent with interpolated values of the isoscape. The interpolated groundwater isoscape is spatially continuous; therefore, in this study it is used as a reference instead of individual direct observations. The mean of 7 direct δ2H groundwater observations (Babre et al., 2016; Raidla et al., 2016) is -78.9% ± 2.2% SD. The linear regression line between observed δ18O and δ2H has a slope of 12 ± 3 SD and an intercept of 55 ± 36 SD; note the high uncertainty.
6 Materials and methods 6.1 Meteorological and hydrological data The meteorological data – temperature and precipitation – was obtained from meteorological stations Rūjiena, Ainaži and Priekuļi (Fig. 1). These stations are 16 to 33, 30 to 50 and 55 to 75 km, respectively, from the study region; their elevations are 6.28, 67.71 and 122.07 m a.s.l., respectively. River Salaca water levels and discharge were measured at two national observation stations (Fig. 1(b)). Meteorological and hydrological observation stations are operated by the national meteorological agency "Latvian Environment, Geology and Meteorology Centre."
6.2 Sampling strategy and methodology The sampling strategy was to observe common water types in the study region using precipitation data and discharge data from Lake Burtnieks, tributaries to the River Salaca, raised
11
bog and groundwater (Fig. 1 and 2). Detailed descriptions are given in Supplementary material – 1.
Fig. 2. Conceptual model of runoff in the upper Salaca catchment: 1 – precipitation; 2 – drainage from raised bogs (LU1, PP1, PV1); 3 – springs discharging in the River Salaca valley (GA1); 4 –confined groundwater presumed to discharge directly into the River Salaca (GU1); 5 – phreatic groundwater draining from till terrain with fine grained cultivated soils (RU1, RU2, RU3); 6 – tributaries Ramata and Iģe (RV1, IV1); 7 – River Salaca channel (SV1, SV2, SV3) mostly originating from Lake Burtnieks.
Samples were collected monthly for 25 consecutive months starting in August 2015, generally at the last days of each month. For two months – July and November 2016 – samples were collected every two days. The first period of intensive sampling corresponds to midsummer conditions with the strongest modulation in surface water isotope values likely due to evaporation. While, the second period was during the transition from summer to winter conditions, and shifting isotopic values in surface waters were expected. EC and pH were measured in the field using a WTW multimeter Multiline 3420 with a TetraCone 925 probe for EC and temperature, and a SenTix 940 probe for pH and temperature at 12
each sampling location. Precipitation was measured when collecting the isotope samples. Samples, including duplicates, were collected in 25 ml high density polyethylene (HDPE) bottles without filtration. River water was sampled with buckets directly from the stream surface and immediately transferred to a sample bottle. Groundwater was sampled from temporary monitoring wells with a 1-m long filter using either submersible or peristaltic pumps. Before sampling, all water was removed from the well so that fresh water flowing into the well could be sampled or pumped until EC and pH values stabilised.
6.3 Laboratory procedures Stable isotope ratios of hydrogen and oxygen in water were analysed in the Laboratory of Environmental Dating at the University of Latvia (Faculty of Geography and Earth Sciences). Isotope ratios are expressed in standard δ-notation relative to the Vienna Standard Mean Ocean Water (VSMOW; Craig, 1961). Both isotope ratios of hydrogen and oxygen were measured using the cavity ring-down laser spectroscopy method (Brand et al., 2009) with a Picarro L2120-i Isotopic Water Analyzer. Reproducibility of stable isotope measurements was less than ±0.1‰ for δ18O and ±1‰ for δ2H. To assure quality of water sampling and processing in the laboratory, internationally-accepted procedures elaborated by the IAEA (Aggarwal et al., 2007) were followed. The laboratory has successfully participated in world-wide open proficiency tests on the determination of stable isotopes in water organised by the IAEA in 2016 (Wassenaar et al., 2018).
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7 Results 7.1
Hydrometeorological conditions The observed precipitation rarely exceeded 50mm/week and was evenly distributed
throughout the year (Fig. 3), approximately corresponding with long-term average observations. One exception occurred in August 2016 when the monthly precipitation at the Rūjiena meteorological station was 233 mm.
Fig. 3. (a) weekly precipitation in Ainaži, Priekuļi and Rūjiena meteorological stations; (b) relative air humidity in Rūjiena meteorological station and (c); air temperature in Rūjiena
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meteorological station and water temperature in the River Salaca (observation site SV3) from August 2015 to September 2017.
During the observation period, summer and early autumn was characterised by low levels of both surface water (Fig. 4) and groundwater tables (Fig. 5); while in winter and spring, water tables were high. At the Mazsalaca hydrological station, the ice cover was present in January 2016 and January to mid-February 2017, reflecting relatively mild winters. It is assumed that a stable water temperature close to 0°C indicates ice cover at SV1 – the headwater of the River Salaca (Fig. 4). According to this indicator, the ice cover in winter 2015/2016 was less than three months, and in 2016/2017, it was around four months. The average duration of ice cover on Lake Burtnieks from 1988 to 2001 was 116 days (Apsīte et al., 2012), almost four months.
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Fig. 4. River Salaca: (a) observed relative water table, (b) electrical conductivity, (c) temperature; (d) δ18O and € d-excess values in the River Salaca at the outlet of Lake Burtnieks (SV1) and downstream locations (Mazsalaca, SV2, SV3 and Lagaste)
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Fig. 5. Tributaries: (a) observed relative water table, (b) electrical conductivity, (c) temperature; (d) δ18O and (e) d-excess values for the small rivers Iģe (IV1) and Ramata (RV1).
7.2 Isotope ratios In total isotope rations were measured in 554 samples. However, several samples were discarded after quality check. Initially, two precipitation traps were set up in accordance with IAEA/GNIP (2014) guidelines; but evaporation, partial shading by vegetation, blocking of tubing by ice and vegetative litter, and a change in methodology compromised the sample quality. Therefore, all the precipitation samples were excluded from the analysis in this paper. Finally, results from detailed observations (two month of measurements every other day) are not examined in this paper as well. 17
Fig. 6. (a, b) observed range of isotope ratios and (c) electrical conductivity; the grey zone in the δ18O plot represents the groundwater isoscape and its uncertainty (Raidla et al., 2016) across the entire catchment.
All samples considered after quality control were within the range of -6.65 to -14.4 δ18O‰ and -60.8 to -100.9 for δ2H‰; d-excess varied between -11.2 and 21.9‰ (Fig. 6). Groundwater samples (GU1, RU1, RU2, RU3) including the spring (GA1) showed more stable results for all parameters throughout the observation period and have the most depleted average values (standard deviations of 0.1 for oxygen-18 and 0.8 for deuterium). Bog samples (PP, PV1, LU1) showed significantly enriched isotope ratios compared to other sampling sites and had the 18
lowest mean deuterium d-excess (1.8‰). Isotope ratios of the samples reflect the local evaporation line, while phreatic groundwater (GA1, RU1, RU2, RU3) and groundwater (GU1) samples mostly coincide with the GMWL (Fig. 7). The lowest mean EC values are from raised bog, indicating atmospheric recharge, while the highest values were obtained from shallow groundwater samples (Fig. 6). The broadest range of EC was for surface water samples (Fig. 6).
Fig. 7. Dual isotope plots of observed δ18O and δ2H values; grey vertical zone is the groundwater isoscape from (Raidla et al., 2016), long diagonal line is GMWL and the short diagonal line with uncertainty is the linear regression from the data points.
Essentially all water types (Fig. 4., 5., 8., 9) except groundwater (Fig. 10) have seasonal fluctuations in isotope ratios. Isotope seasonality can be smooth and pronounced like in the River
19
Salaca and its tributaries (Fig. 4 and 5) or event-driven as in phreatic groundwater and raised bogs (Fig. 8 and 9).
Fig. 8. Phreatic groundwater: (a) observed relative water table, (b) temperature, (c) δ18O, (d) d-excess values and (e) electrical conductivity at the spring (GA1), groundwater monitoring wells (RU1 and RU2) and an agricultural drain outlet (RU3).
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Fig. 9. Raised bog: (a) observed relative water table, (b) temperature, (c) δ18O, (d) dexcess values and (e) electrical conductivity at the shallow monitoring well in raised bog (LU1) and River Pigele within raised bog (PP) and outside of it at a downstream location (PV1).
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Fig. 10. Groundwater: (a) observed (b) δ18O, (c) d-excess values and electrical conductivity (c) at the groundwater monitoring well (GU1).
8 Discussion 8.1 The wet events The most extreme average monthly isotope ratios – January to February 2016, August to October 2016 and February to May 2017 (Fig. 11) – can be linked to meteorological conditions.
22
Fig. 11. Average oxygen isotope ratios in different water types. Three events of enhanced groundwater recharge are highlighted: 1. – thaw (melting snow) in January 2016; 2. – strong precipitation in August 2016 followed by relatively dry and warm September; 3. – thaw (snow melt) in February 2017 followed by a warm and dry March and April.
8.2 Winter thaw January 2016 At the end of January 2016, after several weeks of freezing temperatures, a thaw event occurred. In the tile drainage (RU3), strongly depleted water with low EC was observed (Fig. 8), indicating preferential infiltration of snowmelt. To a lesser extent, this depletion signal was evident in tributaries and, with a one-month delay in the raised bog and Salaca River (Fig. 4, 5 and 9).
8.3 Extreme precipitation in August 2016 In August 2016, exceptionally high precipitation was observed (monthly total of 233 mm and 150 mm in a single event on August 23 at Rūjiena meteorological station, Fig. 3); that is 23
approximately one-quarter of the yearly mean precipitation (Briede and Koreļska, 2018). September 2016 had moderate precipitation (28.8 mm, Rūjiena meteorological station), the average air temperature was 13°C and the average relative air humidity was 84% (Fig. 3). The mean phreatic groundwater δ18O and d-excess in August 2016 were -9.7‰ and 3.5‰, respectively (Fig. 11), indicating enrichment by evaporation. Following this event, enriched δ18O was observed in the River Salaca and raised bog as well. A spike of enriched water at the end of August 2016 was found in the River Iģe as well, but not in the River Ramata (Fig. 5). Enriched water averaging δ18O = -10.7‰ (Fig. 8) was observed in Govs Ala spring (GA1) up to three months after this event, while the two-year mean was -11.4‰ with a standard deviation of 0.33. The spring discharges from a sandy phreatic aquifer as evident from the outcrop of poorly consolidated sandstones covered by glacial outwash sediments (State Geological Survey, n.d.). As determined from the topographical map, the thickness of unsaturated sediments above the spring discharge site is around 16 m. The observed excursion of isotope ratios can be explained by preferential infiltration of an unusually large volume of precipitation in this event (Fig. 3).
8.4 Thaw in February 2017 Phreatic groundwater (RU1, RU2, RU3) and Lake Burtnieks (SV1) samples collected at the end of February 2017 had noticeably depleted δ18O values, while water in the tributaries and downstream sampling sites on the River Salaca (SV2 and SV3) had enriched values. Enriched δ18O values were already observed in phreatic groundwater and surface runoff (Fig. 4., 5., 8., 9.) at the end of March 2017. The δ18O enrichment is associated with decreasing d-excess values indicating modification by evaporation. Enriched water was observed in phreatic groundwater samples up to three months after the event (Fig. 11). The same pattern was observed in the raised bog, but with a one-month delay (Fig. 11). 24
It appears that in February 2017, due to the combination of snowmelt and precipitation, a large volume of depleted water appeared in the catchment. Some of it was drained as surface runoff, some entered the groundwater system and some remained on the frozen soil surface. The average air temperature in March 2017 was 1.4 °C and the average relative air humidity was 82% (Fig. 3), thus facilitating evaporation. It is likely that water stagnating on the soil surface attained increasingly enriched isotope values due to evaporation, and as frozen soil was thawing, infiltrated the groundwater and drained into streams.
8.5 Generalisation and implications We suggest that at these three episodes – the impoundment of the water on the soil surface and subsequent infiltration of water enriched by evaporation (Fig. 12) – are the dominant mechanisms producing the observed excursions in isotope ratios (Fig. 11). We suggest that the intensity-distribution of precipitation has significant control on surface and ground water isotope ratios in terrestrial conditions. Enriched water-derived isotope ratios and decrease of d-excess in paleorecords may be due to increased precipitation and its intensity rather than a shift towards more arid conditions. Isotope seasonality is controlled by the complex interplay between precipitation, evaporation, transpiration and hydrological conditions. Impoundment of precipitation water on land surface can occur when rapid snowmelt takes place over frozen ground, when soil permeability is insufficient to drain incoming precipitation or when the groundwater table reaches the soil surface. During events in spring 2016 and 2017 rapid infiltration of precipitation and snowmelt water likely was precluded by froze soil. The event in August–September 2016 likely is combination precipitation intensity exceeding the soil infiltration capacity and groundwater table reaching soil surface at places.
25
Described events are associated with decreasing EC in the River Salaca and its tributaries (Fig. 4 and 5), indicating input of unaltered precipitation water. The magnitude of EC fluctuations is in line with those previously observed (Rodinovs and Kļaviņš, 1993).
Fig. 12. Formation of isotopically-enriched groundwater due to water accumulation in puddles on frozen soil, if average d-excess in precipitation is 10‰.
Alternatively, evolution of isotope ratios can be explained by mobilisation of enriched water stored in topsoil (Sprenger et al., 2016) due to sublimation of the snowpack (Stichler et al., 2001) and infiltration of melt water, or by evaporation of rain or snow trapped by the canopy before reaching the soil and groundwater (Vystavna et al., 2018), or precipitation water may already have a particularly-low d-excess. In addition, enhanced surface runoff can trigger mobilisation of isotopically-enriched water stored in bogs and ponds. Potentially, this can have 26
noticeable effects on isotopic signatures in rivers, as the estimated wetland area in this catchment is between 5% (European Environment Agency, 2016) and 15% (Briede and Rodinovs, 1993). Meteorological conditions that can lead to water impoundment on land surface and cause significant enrichment before entering the subterranean and surface runoff system seem to occur frequently. In Estonia, just north of the study area, during the warm season, there were up to four days of extreme precipitation (exceeding the 99th percentile, R99p, > 20 mm/day). Due to climate change, the number of such extreme events is expected to increase (Päädam and Post, 2011), thus changes in precipitation intensity distribution alone can affect the isotope ratios of groundwater. Kriauciuniene et al. (2012) indicated that the runoff of Salaca River is primarily generated by precipitation implying surface runoff or short-cycle groundwater runoff. Given the predominantly lowland plain type of terrain, that does not favour swift surface runoff, the River Salaca catchment has favourable conditions for isotopic enrichment of precipitation water by the mechanism proposed. We can speculate that in other regions of similar dominant mechanism of river runoff generation mechanisms identified by (Kriauciuniene et al., 2012) may indicate areas where the proposed mechanism is an important factor contribution to isotope ratios of surface and groundwater. Additional studies are clearly needed to assess whether evaporative enrichment of water temporarily residing on the land surface is a significant mechanism contributing to the groundwater isotope ratios. We suggest that water isotope ratios δ18O and δ2H are valuable indicators for groundwater monitoring in line with the EU Water framework directive (The European Parliament and the Council of the European Union, 2000) to better understand evolving changes in recharge conditions and surface-groundwater interactions. Water stable isotopes already have 27
been used as indicators of groundwater recharge sources (Joshi et al., 2018) or to explain variations of groundwater quality (Sarkar et al., 2017). Free-flowing springs like Govs Ala spring (GA1) are often included in groundwater monitoring programmes (LVĢMC, 2015b) as a costeffective means of obtaining groundwater samples without the need for heavy pumping equipment. Episodic discharge of preferential infiltration as indicated by a spike in isotope ratios in August to October 2016 (sampling site GA1, Fig. 8) can complicate or even disable meaningful interpretation of results. Water quality parameters measured during such episodes would be difficult if not impossible to interpret and to generalise over a wider region.
9
Conclusions We found characteristic signatures of isotope ratios for each type of water studied: water
in raised bogs, confined groundwater, phreatic groundwater in terrains with fine-grained cultivated soils and forested sandy soils, and river and lake water. On average the most enriched water was found in raised bog (δ18O = -8.84 ‰) and river emanating from Lake Burtnieks (δ18O = -9.18‰). The most depleted water was found in a spring discharging phreatic groundwater at a forest site with sandy soil (δ18O = -11.4‰). We suggest that evaporation of water impounded on the soil surface is an important mechanism leading to isotopic enrichment of surface and subsurface water. Further case studies are needed to test if this is a regionally significant mechanism contributing to the surface and groundwater isotope ratios.
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10 Acknowledgements Funding: This work was supported by the Latvian National Research Programme EVIDEnT (No. 10-4/VPP-2/19), subproject 5.2 and grant No. ZD2016/AZ03 "Climate change and sustainable utilization of natural resources” at the University of Latvia. The data set is available as Data in Breif publication “Water stable isotope data set in temperate, lowland catchment, two years of monthly observations”.
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Highlights 1. Monthly water isotope observations for two years at 13 sampling points in a temperate catchment 2. Distinct water isotope signatures in raised bog, groundwater and surface water 3. Surface water isotope seasonality due to evapotranspiration rather than precipitation 4. Isotope enrichment of water impounded on soil surface is propagated to the groundwater and streams
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