Lacustrine Groundwater Discharge Research Articles

Spatio-temporal variations of lacustrine groundwater discharge and related nutrient fluxes in a typical lake in front of hillocks

Lacustrine groundwater discharge (LGD) as a hidden hydrologic process has been widely found to affect water and nutrient balance in various lakes, but a comprehensive research on both spatial and temporal variations of LGD and associated nutrient fluxes was poorly conducted. Herein, this study elucidated their spatio-temporal variations and underlying factors of a typical lake in front of hillocks with spatially variable topography and hydrogeology, and intense seasonal groundwater-lake exchange, within central Yangtze River basin. The 222Rn mass balance model was used in conjunction with hydrochemistry, water isotope, and nutrient data. The results showed that the overall LGD rate and related N/P fluxes were clearly higher in the wet season than in the dry season, for which the stronger evaporation relative to precipitation, more groundwater recharge from agricultural irrigation or aquacultural pond infiltration, and more N/P input from anthropogenic or aquacultural activities could be responsible. In the wet season, the area with more recharge from irrigating water or fish pond water infiltration and associated extensive agricultural or aquacultural activities, had the significantly highest LGD rate and related N/P fluxes. In the dry season, the area with the deepest water depth and largest topographic slope, exhibited the significantly highest LGD rate and slightly higher LGD-related N/P fluxes due to relatively low groundwater N/P concentrations. This study enriched the understanding on LGD and related nutrient fluxes in the perspective of both spatial and temporal variations, taking a new type of lakes (i.e., lakes in the transitional zone of hills and plains in temperate climate) as a typical case.

Seasonal characteristics of groundwater discharge controlled by precipitation and its environmental effects in a coal mining subsidence lake, eastern China

Many regions have formed subsidence lakes due to underground mining in the world. However, seasonal variations of lacustrine groundwater discharge (LGD) rate and solute fluxes in the coal mining subsidence were rarely reported. In this study, we conducted four seasonal samplings in a coal mining subsidence, during which samples for stable water (δ18O) and radioactive (222Rn) isotopes were collected to quantify the seasonal dynamics of LGD rates. The LGD rates estimated from the 222Rn mass balance model were 10.2 ± 8.7, 5.5 ± 3.2, 11.5 ± 7.8, and 7.8 ± 4.5 mm d−1 in summer, autumn, winter and spring, respectively. According to the 18O mass balance model, the corresponding LGD rates were 15.1, 7.3, 15.6, and 11.3 mm d−1 in summer, autumn, winter and spring, respectively. We found a significant correlation between precipitation and LGD rates, suggesting precipitation was recognized as the main control factor for seasonal variations of LGD rates. Based on this correlation, the extrapolated LGD rates over a year ranged from 3.1 to 12.7 mm d−1 with an average of 8.8 mm d−1. Moreover, the fluxes of dissolved silicon (DSi), iron (Fe), and manganese (Mn) from LGD in autumn were (1.6 ± 0.9) × 105, (1.9 ± 1.1) × 104, and (1.1 ± 0.6) × 104 mol a−1, respectively. Correspondingly, in winter they were (3.5 ± 2.4) × 105, (4.1 ± 2.8) × 103, and (2.8 ± 1.9) × 103 mol a−1, respectively. This study demonstrated significantly seasonal variations of LGD, with precipitation being the main control factor of LGD in the coal mining subsidence lake. The fluxes of dissolved substance (DSi, Fe, Mn) from LGD need to be emphasized because they may have important impacts on the ecological stability in coal mining subsidence lakes.

Evaluating heterogeneous lacustrine groundwater discharge of Hulun Lake using radium isotopes in Inner Mongolia, China

Study regionHulun Lake Basin, Northern China Study focusUnderstanding lacustrine groundwater discharge (LGD) is important for elucidating the hydrological cycle and sustainable use of water resources. Hulun Lake is the largest fresh water lake and has prominent ecological function in North China. However, the effect of groundwater discharge on the water balance and spatial distribution of LGD under different aquifers have not yet been evaluated in detail. Herein, radium (223Ra, 224Ra, and 228Ra) and stable isotopes (δD, δ18O) were used to identify the LGD locations and quantify the spatial LGD flux into the lake. New hydrological insights for the regionThe Ra isotope activities in groundwater from the fissured bedrock in the western part of the Lake are depleted relative to those from the east, underlain by the Quaternary unconsolidated sediments aquifer overlying granitic rocks. The spatial distribution of LGD under heterogeneous aquifers was revealed and groundwater discharge from fractured bedrock aquifer is the predominant source of the lake. The LGD flux is estimated to range from (1.07 ± 1.36)× 107 to (6.54 ± 1.89)× 107 m3/d, which exceeds that in the Quaternary aquifer by one order of magnitude due to the high permeability enlarged via tectonic faults and fractures. These findings promote scientific understanding of water balance in Hulun Lake and are instructional for LGD studies of freshwater lakes with complex water systems in cold and arid regions.

Tracing groundwater discharge into a coal mining subsidence lake in eastern China: Observations from water stable (δD and δ18O) and radon (222Rn) isotopes

Many coal mining subsidence lakes have formed in eastern China due to underground coal mining. Lacustrine groundwater discharge (LGD) is of great importance to the hydrological cycle and the eco-environment of lakes. However, LGD in coal mining area is rarely reported. In the study, we quantified groundwater discharge into a Huainan coal mining subsidence lake (China) using water stable (D and 18O) and radioactive (222Rn) isotopes. 222Rn and 18O mass balance models were used to independently estimate LGD. The lake water was sampled based on the horizontal and vertical profile sampling. The isotope depth profiles were weighted according to the lake bathymetry to obtain a representative isotope inventory by geographic information system (GIS) analysis. The LGD rates estimated from two models were comparable, and the values were 18 mm d−1 for the 18O and 14 ± 8 mm d−1 for the 222Rn. Compared with the LGD of Ammelshainer See lake (Germany), a subsidence lake formed by coal mining, the LGD in the study was larger, which may be related to the different hydrological conditions in different areas. By integrating the LGD rates from these two models, this study quantitatively analyzed the role of groundwater in maintaining the water balance in subsidence lakes, which is of great significance to water resources assessment and sustainable utilization of coal mining subsidence lakes in eastern China.

Quantification of groundwater-borne greenhouse gases (CH4, CO2, N2O) fluxes to an oxbow lake in a subtropical alluvial-lacustrine plain

Aquatic ecosystems are an important source of greenhouse gases (GHGs) released to the atmosphere. However, studies on GHGs fluxes from lacustrine groundwater discharge (LGD) remain limited, particularly for subtropical alluvial-lacustrine plains. This study used the radon (222Rn) mass balance model to quantify seasonal variations in LGD rates and fluxes of groundwater-borne GHGs (CH4, CO2, N2O) to the Tian-E-Zhou oxbow lake in Jianghan Plain, central Yangtze. The results showed that the LGD rate in winter was 57.67 ± 28.37 mm/d which was higher than that in summer (24.72 ± 12.16 mm/d). The groundwater-borne fluxes of CH4, CO2, and N2O into the lake in winter were 7.84 ± 6.81 mmol m−2 d−1, 1.47 ± 1.07 mmol m−2 d−1, and 3.50 ± 1.90 × 10−5 mmol m−2 d−1, respectively, whereas that in summer were 1.48 ± 2.36 mmol m−2 d−1, 0.72 ± 0.47 mmol m−2 d−1, and 1.53 ± 1.00 × 10−5 mmol m−2 d−1, respectively. High groundwater-borne fluxes of CH4 across both winter and summer could be attributed to abundant buried organic carbon and strong groundwater reducing environment in this subtropical alluvial-lacustrine plain. Seasonally, fluctuations in water levels mainly affected LGD rates, further resulting in greater groundwater-borne GHGs fluxes in winter than in summer. This study can act as an important reference for future studies on the role of groundwater as an emission pathway for GHGs in lakes of subtropical alluvial-lacustrine plains.

Spatial variability of lacustrine groundwater discharge in the largest urban lake in Asia: Coupled influence from land use and hydrogeology

AbstractLacustrine groundwater discharge (LGD) has a profound impact on the status of lake ecosystems. The distinct spatial variability of LGD has been acknowledged, but corresponding research has rarely been conducted, especially in urban lakes with changing land use. In this study, the spatial variability of LGD in Tangxun Lake, the largest urban lake in Asia, was quantitatively estimated using the 222Rn mass balance model. Catchments with different land use and hydrogeological conditions within the study area are divided into five regions. The results revealed three ranges of the LGD rate in five regions: the first range of 25 mm/day (region II: 27.25 ± 8.38 mm/day and region IV: 23.04 ± 7.51 mm/day), the second range of 15 mm/day (region III: 14.16 ± 5.64 mm/day), and the third range of 10 mm/day (region I: 10.61 ± 4.31 mm/day, region V: 9.05 ± 3.60 mm/day). Here, we found that the LGD rate of urban lakes was largely controlled by land use patterns, although it was also inherently affected by hydrogeological conditions, which is consistent with a significant positive correlation between the proportion of impermeable/total surfaces within 1 km of the lakeshore and the LGD rate of the corresponding region. The proposed reason is that the head difference between groundwater and lake water decreases with increasing proportion of impermeable surfaces around the urban lake because of reduced infiltration, leading to the decrease of LGD rate. This study provides new insights into the spatial variability of LGD for lakes in a changing environment.

Estimation of Lacustrine Groundwater Discharge (LGD) to an urban Himalayan lake using environmental tracers (222Rn, δ18O, EC)

Lacustrine Groundwater Discharge (LGD) is a complex process that controls lake characteristics and plays a dominant role in modifying the hydrological and chemical budget of a lake. The current study was carried out with the aim to assess the groundwater discharge component of the hydrological budget of an urban Himalayan lake. We used natural tracers including radioisotope of radon (222Rn), stable water isotopes (δ2H and δ18O), electrical conductivity and water temperature to identify the potential groundwater discharge zones in a lake. The obtained LGD results, based on radon mass balance model (RMB) and end member mixing analysis (EMMA) of stable water isotopes in combination with physical parameters, show temporal as well as spatial variation. The total LGD to the Dal Lake, estimated from RMB, was 62.25 ± 9 × 104 m3/d (24 ± 3 %) and 33 ± 6 × 104 m3/d (36 ± 4 %) during high and low flow periods, respectively. The results suggested that the stream water is the dominant contributor accounting for 75 % (±7) and 55 % (±8), followed by groundwater as 22 % (±5) and 40 % (±4), during high and low flow periods, respectively. However, the contribution from direct precipitation is very little (3–4 %). High groundwater flux towards northeastern side of the lake is substantiated by more aquifer thickness and high hydraulic gradient. The current study offers first in-depth understanding about LGD of an urban Himalayan lake and could be applicable for lake ecosystem protection policies and water resource management plans in the region and elsewhere.

Contrasting nutrients input along with groundwater discharge to east Dongting Lake, central China: A geological perspective

The spatial patterns of lacustrine groundwater discharge (LGD) and associated nutrient inputs are crucial for effective management and protection of lakes. Multiple factors have been found to influence the spatial differences in LGD rates and associated nutrients loads, but the influence of geological conditions on the differences have not been well understood. In this study, we quantified LGD rates and associated nutrients loads on the west and east of the lake with contrasting geological conditions of East Dongting Lake (EDL) within central Yangtze catchment and discuss the influence of geology on the spatial differences, through 222Rn mass-balance model, water chemistry coupled with existing geological data. The results showed that LGD rates were 34.76 ± 23.36 mm/d in the east EDL which is characterized by hilly geomorphy, deep/fast/narrow flowing, coarse-grained lakebed and large hydraulic gradients (0.004–0.006). Surprisingly, LGD rates were higher (71.47 ± 52.16 mm/d) in the west EDL which is characterized by alluvial-lacustrine plain geomorphology, shallow/sluggish flowing, clayey or silty lakebed and low hydraulic gradients (0.0002–0.0015). The remaining factor determining the higher LGD rates in the west EDL is the permeability of the porous aquifer connected with the lake. The groundwater around the east EDL existed in a less confined environment, and frequent flushing led to low concentrations of nutrients. By contrast, rapid burial of sediments and deposition of paleo-lake sediments since Last Deglaciation formed an organic-rich and reducing environment, which facilitated the enrichment of geogenic nutrients. As a result, the loads of LGD-derived nutrients in the west generally exceeded that in the east by one order of magnitude. The study provides new understanding of the spatial differences in LGD and associated nutrients input to the large lakes resulting from variations in geological conditions, thus serving as a reference for ecological protection of lakes.

Spatial Patterns and Quantification of Lacustrine Groundwater Discharge Determined Based on 222Rn

AbstractLacustrine groundwater discharge (LGD) is of great importance to the hydrological cycle and the eco‐environment of lakes. LGD is often characterized by large spatial variability, but the spatial patterns of LGD have rarely been quantified, particularly at large whole‐lake scales. This study presents quantitative estimation of the spatial patterns of LGD in an oxbow lake in the central Yangtze River Basins by a segmented application of 222Rn mass balance model. The results show that the LGD rates in all points ranged from 10.05 to 197.25 mm/d, with an average of 61.23 mm/d. Larger LGD rates were not only occurring in the lakeshore area, but also in the lake center. The high LGD rate in the lake center area here was likely a result of a direct connection between the water‐rich confined aquifer and the lake. Sensitivity analysis of the 222Rn mass balance show that lake water 222Rn concentration, groundwater concentration, wind speed and water depth were sensitive parameters, which were important parameters in controlling the quantitative results of LGD. The average LGD rate uncertainty and percentage were 28.45 mm/d and 49.20%. This study thus provides a solution in quantifying the spatial patterns of LGD lakes.

Dominance of evaporation on lacustrine groundwater discharge to regulate lake nutrient state and algal blooms

As global threats to freshwater lakes, eutrophication and harmful algal blooms (HABs) are governed by various biogeochemical, climatological and anthropogenic processes. Groundwater is key to join these processes in regulating HABs, but the underlying mechanisms remain unclear. Here, we leveraged basin-wide field data of Lake Taihu (China's largest eutrophic lake) and global archives, and demonstrate the dominance of evaporation on lacustrine groundwater discharge (LGD) in shallow lakes. We extrapolated decadal LGD and the derived nutrient loadings and found that HABs promptly consume ubiquitous groundwater borne nutrients, leading lake water N: P ratios 2–3 months time lagged behind LGD N: P ratios. We conclude that evaporation dominated LGD is an unraveled but crucial regulator of nutrient states and HABs in shallow lakes, which advocates synergistical studies from both climatological and hydrogeological perspective when restoring lake ecosystems.

Deep hydrothermal and shallow groundwater borne lithium and boron loadings to a mega brine lake in Qinghai Tibet Plateau based on multi-tracer models

Brine lakes are good natural laboratories to investigate groundwater influences on the hydrologic and chemical evolutions in arid environments, and the mineralization processes under intensive evaporation. Lacustrine groundwater discharge (LGD) is the vital conveyor for the loadings of resource elements in the brine lakes. Da Qaidam Lake, located in the Qaidam basin of the Qinghai–Tibet Plateau (QTP), is one of the largest brine lakes for boron and lithium resources in China. Lithium and boron in the lake are considered to be dominantly sourced from deep hydrothermal groundwater and shallow groundwater, but the partitioning of deep and shallow components to the lake and the derived lithium and boron loadings remain unknown, LGD derived boron and lithium provide the primary source of the salt lake. vitally regulates the formation, evolution and mineralization of Li and B resources in the brine lake. This study performs systematical investigations of radium isotopes (226Ra, 228Ra, 224Ra and 223Ra), lithium, boron, and other hydrogeochemical parameters in different water endmembers around the brine lake. The results indicate that radium isotopes are significantly enriched in the hydrothermal groundwater and will be removed by co-precipitation with barite precipitates in the lake water. The multi-tracer models coupled radium bass balance, conservative tracer buildup and water budget were deployed to precisely constrain radium co-precipitation rates, and to quantify the deep and shallow LGD (total LGD = LGDD + LGDS) and the derived lithium and boron loadings. Radium co-precipitation coefficient is obtained to be 4.7–6.1 y−1. LGDD and total LGD are estimated to be 8.8 × 106 and 3.3 × 107 m3 y−1, respectively, which account for 11.9% and 57.2% of the total water input. LGDD and total LGD derived lithium/boron loadings constitute up to 70.2/60.1%, and 79.0/77.7% of the total loadings, respectively, indicating the significance of disproportionate LGDD in delivering resource elements into the brine lake. This study presents the first attempt to partition the deep hydrothermal and shallow LGD to a mega the QTP brine lake by multi-tracer models and the findings contribute to the understanding of lithium and boron budgets in the brine lakes of the QTP and worldwide.

Evaluating lacustrine groundwater discharge to a large glacial lake using regional scale radon‐222 surveys and groundwater modelling

AbstractLacustrine groundwater discharge (LGD) can be an important pathway for delivering pollutants to lakes but this pathway is often poorly characterized. Evaluating the potential impact of LGD on lake water quality requires understanding the magnitude and spatial variability of LGD, as well as understanding the age and flow paths of the discharging groundwater (e.g., recharge area, groundwater flow paths, and travel times). This study first compares LGD rates along two ~40 km shoreline lengths of a large glacial lake, Lake Simcoe, Canada, that were independently estimated via a radon‐222 (222Rn) field survey and via regional scale groundwater‐surface water modelling. Backward particle tracking analysis is then used to examine the age and flow paths of the LGD and thereby assess the potential for the LGD to deliver anthropogenic pollutants to the lake. The field and modelling results compare well with respect to the magnitude and spatial variability of LGD. However, the comparison highlights the need for well‐defined hydrogeological characterization if regional scale models are to be applied for LGD estimation. The particle tracking analysis indicates large variation in the groundwater flow path lengths and travels times (>1000 years to <50 years) for LGD along the shoreline. This illustrates that the LGD along different shoreline areas has varying potential to deliver anthropogenic pollutants to the lake. The study findings demonstrate the benefits of comparing independent field measured and model‐simulated LGD estimates, and moreover suggest that it may be possible, in some cases, to use existing regional scale groundwater‐surface water models, purpose‐built for other water resource and quality objectives, to conduct preliminary evaluation of LGD contributions to lakes. Preliminary model‐based evaluation would enable field efforts aiming to quantify and manage LGD to be better targeted rather than relying solely on regional scale field techniques that are often highly resource intensive.

Contribution of groundwater discharge and associated contaminants input to Dongting Lake, Central China, using multiple tracers (222Rn, 18O, Cl-).

Lacustrine groundwater discharge (LGD) can play an important role in water and contaminant mass balance of lakes. Dongting Lake is the second largest fresh lake in China which is connected to Yangtze River and has quite prominent ecological status and function within Yangtze River basin. However, the effect of groundwater discharge on the balance of water and contaminant in Dongting Lake has long been overlooked. This study estimated the groundwater discharge and associated contaminants input into Dongting Lake during the dry season using multiple tracers (222Rn, 18O, Cl-). After sensitivity analysis of different models, it is found that the result of 222Rn mass balance model is the most reliable. Based on the 222Rn mass balance model, the groundwater discharge rate is estimated to be 73.94mm/d and the contribution of LGD to water balance is 10.94%. As the main nutrient components, NH3-N, P and Si from groundwater input account for 23.65%, 5.12% and 30.15% % of the total input, respectively. As the main heavy metal components, Fe, Mn and As from groundwater input all account for more than 50% of the total input. Although the LGD rate is relatively small, the contaminant input from LGD is significant enough, which may be a potential threat to the ecological stability of Dongting Lake. In this study, the mass balance models of multiple tracers were integrated to understand the role of groundwater in maintaining the water balance and pollution status of Dongting Lake, which has certain reference significance for the LGD study in plain lakes or reservoirs with complex water systems in humid regions.

Lacustrine Groundwater Discharge Through Giant Pockmarks (Lake Neuchatel, Switzerland)

Pockmarks are circular depressions on the floor of oceans and lakes and constitute potential hot spots of gas ebullition and/or groundwater discharge. Marine pockmarks are well studied, whereas lacustrine pockmarks are virtually unexplored. In Lake Neuchatel (Switzerland) four giant pockmarks of 80 to 150 m in diameter are located along the northern shore and adjacent to the karst system of the Jura Mountains. Two pockmarks have a ~60 m-deep chimney filled with mud; two are funnel-shaped 12 m and 29 m deep holes. We present evidence for the presence of groundwater in the pockmark chimneys and active lacustrine groundwater discharge (LGD) at both pockmark types. Temperature, electrical conductivity and calcium concentrations of the pore water in the chimneys show values typical for karst water (maximal sampling and profiling depth: 41 m into the pockmark) and contrast strongly with the properties of the lake water. TOC and TIC indicate that the chimney mud is homogenized and composed of liquefied sediments from the entire deglacial to Holocene lacustrine sediment succession. Mini mud volcanoes apparent on the suspension surface imaged with a remotely operated vehicle (ROV) localize the groundwater exit points and confirm LGD. LGD is further corroborated by electrical conductivity anomalies detected above the lutoclines and within a funnel-shaped pockmark during the ROV survey. We conclude that the giant pockmarks in Lake Neuchatel represent a type of subaquatic springs that connect the water body of the lake with the karst system. A next essential research step will be to quantify LGD via the pockmarks in order to assess their lake-wide relevance. This study underlines the existing need for research on the connectivity of lakes and oceans with groundwater systems for completing our understanding of the hydrological cycle.

Hydrogeological controls on heterogeneous groundwater discharge to a large glacial lake

Lacustrine groundwater discharge (LGD) can be an important pathway for delivering pollutants to large lakes but this pathway is often neglected. This is in part because characterizing the magnitude and spatial variability of LGD to large lakes is highly resource intensive. To better target efforts, particularly in glacial environments where the hydrogeology is often highly complex, there is a need to understand linkages between spatial LGD patterns and hydrogeological features. The objective of this study was to evaluate the magnitude and spatial variability of nearshore LGD to Lake Simcoe (area = 722 km2) and use this information to explore hydrogeological controls on the observed spatial LGD patterns. LGD was quantified by applying a steady state mass balance model to radon-222 (222Rn) data obtained from boat surveys conducted along ~80% of the lake’s shoreline. Overall, LGD was highly spatially variable with total LGD to Lake Simcoe estimated to range from 7 ± 1% to 16 ± 2%, respectively, of the total volume of tributary inputs during summer. No large regional scale differences in LGD were found despite the diverse hydrogeological conditions around the lake. Rather, discrete shoreline areas with high LGD were observed around the lake. Areas with high LGD were related to permeable nearshore surficial sediments and tunnel channel aquifers which provide connectivity between large recharge features (moraines), regional aquifer units, and the lake. The study findings are needed to inform actions aimed at managing non-point pollution sources to large glacial lakes including LGD.

Evaluation of lacustrine groundwater discharge and associated nutrients, trace elements and DIC loadings into Qinghai Lake in Qinghai-Tibetan Plateau, using radium isotopes and hydrological methods

Qinghai Lake, the largest lake in China with a surface area of 4260 km2, lies in the northeastern Qinghai-Tibet Plateau (QTP) at a mean altitude of 3194 m above sea level. The lake has a water volume of 71.6 × 109 m3, a salinity ranges of 4.18 ± 0.14 ppt to 12.3 ± 0.14 ppt during sampling period which is about a half of typical seawater. The aquifer systems at the lake shoreline have obvious saline water and freshwater mixing zones with similar hydrogeological and biogeochemical settings to coastal aquifers. This study aims to estimate lacustrine groundwater discharge (LGD) and the associated chemical loadings to western Qinghai Lake. Naturally occurring radium isotopes (223Ra, 224Ra, 226Ra and 228Ra), nutrients, heavy metals and dissolved inorganic carbon (DIC) were systematically investigated in different water endmembers of western Qinghai Lake. LGD was estimated with radium inventory model and radium eddy diffusion model, leading to a LGD estimate of 7.67 × 106 m3 d−1 and 8.52 × 106 m3 d−1, respectively. The fresh groundwater discharge obtained based on regional water budget is about 5.97 × 106 m3 d−1 during sampling period, accounting for 73.74% (ranging from 53.22 to 80.43%) of total LGD. This study reveals that the fresh groundwater component is dominant in LGD of Qinghai Lake, which is quite different from most coastal scenarios. Furthermore, the chemical loadings derived from fresh-LGD and recirculated-LGD are evaluated. The preliminary results indicate that fresh-LGD is an important source of nutrient, barium, strontium, uranium, and DIC to the lake, and may significantly impact biogeochemical and ecological processes in the lake. This study presents the first LGD estimate in the largest QTP saline lake in China, and would be instructive to further similar studies in saline lakes in QTP and elsewhere in the world.

Groundwater discharge gives periphyton a competitive advantage over macrophytes

Lacustrine groundwater discharge (LGD) can significantly contribute to nutrient budgets of lakes. LGD usually occurs in shallow littoral zones, and thus, potentially affects periphyton and macrophytes. These benthic primary producers asymmetrically compete for resources, with periphyton and rooted macrophytes having an advantage in either light or sediment nutrient acquisition, respectively. LGD can advectively transport sediment phosphorus (P) into the water, supporting periphyton growth. We hypothesised that above a threshold level of sediment P concentrations, negative effects of periphyton shading exceed the positive effects of sediment P on macrophytes. We performed laboratory experiments comparing the growth of periphyton and a macrophyte (Stuckenia pectinata) at four different sediment P concentrations with and without simulated LGD. In LGD treatments, P mobilized from sediment promoted periphyton growth, and macrophyte growth rates negatively correlated with periphyton shading when experiments were performed with young plants. Macrophytes died when shading levels exceeded 75% of incident light. Our results confirm that LGD can significantly affect the competition between benthic primary producers at the expense of macrophytes. This process is supposed to be most relevant in oligotrophic water bodies or during periods of P deficiency in the open water, when advective transport of sediment P by LGD can prevent nutrient limitation in periphyton.

Evaluation of lacustrine groundwater discharge, hydrologic partitioning, and nutrient budgets in a proglacial lake in the Qinghai–Tibet Plateau: using <sup>222</sup>Rn and stable isotopes

Abstract. Proglacial lakes are good natural laboratories to investigate groundwater and glacier dynamics under current climate conditions and to explore biogeochemical cycling under pristine lake status. This study conducted a series of investigations of 222Rn, stable isotopes, nutrients, and other hydrogeochemical parameters in Ximen Co Lake, a remote proglacial lake in the east of the Qinghai–Tibet Plateau (QTP). A radon mass balance model was used to quantify the lacustrine groundwater discharge (LGD) of the lake, leading to an LGD estimate of 10.3±8.2 mm d−1. Based on the three-endmember models of stable 18O and Cl−, the hydrologic partitioning of the lake is obtained, which shows that groundwater discharge only accounts for 7.0 % of the total water input. The groundwater-derived DIN and DIP loadings constitute 42.9 % and 5.5 % of the total nutrient loading to the lakes, indicating the significance of LGD in delivering disproportionate DIN into the lake. This study presents the first attempt to evaluate the LGD and hydrologic partitioning in the glacial lake by coupling radioactive and stable isotopic approaches and the findings advance the understanding of nutrient budgets in the proglacial lakes of the QTP. The study is also instructional in revealing the hydrogeochemical processes in proglacial lakes elsewhere.

Thermal infrared imaging for the detection of relatively warm lacustrine groundwater discharge at the surface of freshwater bodies

Thermal infrared (TIR) imaging has been previously applied to survey relatively large thermal footprints in coastal zones, lakes, reservoirs and rivers. In freshwater systems, the buoyancy of relatively warm groundwater during the winter months allows for the surface identification of groundwater discharge or thermal pollution using TIR imaging. However, information regarding the performance of TIR for resolving this warm groundwater upwelling is limited, particularly at fine spatial scales and variable discharge rates. In order to evaluate the suitability of TIR to trace warm groundwater upwelling at the water surface of lakes, ponds and reservoirs (e.g. lacustrine groundwater discharge (LGD) in shallow near-shore zones) we conducted a mesocosm experiment with a TIR camera situated 4 m above the water surface to capture thermal patterns in response to different groundwater discharge rates, weather conditions and the diurnal cycle. A fiber optic distributed temperature sensing system (FO-DTS) installed at 2 cm below the water surface was used to ground-truth spatial patterns observed in TIR images. Results show the impacts of both the diurnal cycle of net radiation and prevailing weather conditions on the accuracy of TIR imaging for resolving warm groundwater discharge. Most reliable results were obtained under overcast weather conditions and during the night. The results of our study provide guidance for those looking to use TIR for conducting thermal tracing of LGD at the surface of freshwater bodies during winter.

How does the groundwater influence the water balance of a lowland lake? A field study from Lake Stechlin, north-eastern Germany

The subsurface catchment of Lake Stechlin is quite complex. Water level measurements in groundwater wells with different screen depths show that groundwater in the lake’s catchment may partly discharge to the lake and partly leak to deeper aquifers. Simple steady-state 2D vertical modelling of eight different scenarios (different leakage areas, different hydraulic conductivities) was conducted in order to explore the local and regional groundwater flow system around Lake Stechlin and the origin and possible magnitude of lacustrine groundwater discharge. A comparison of the different scenarios with in situ measurements of lacustrine groundwater discharge by temperature sticks and seepage meters revealed that only a part of the groundwater recharge in the catchment discharges to the lake while the rest leaks into deeper aquifers. This implies a differentiation of the subsurface catchment into a local and a regional flow system and impacts on travel times and chemical reaction time scales.