Identifying, characterizing and predicting spatial patterns of lacustrine groundwater discharge

Abstract

Abstract. Lacustrine groundwater discharge (LGD) can significantly affect lake water balances and lake water quality. However, quantifying LGD and its spatial patterns is challenging because of the large spatial extent of the aquiferlake interface and pronounced spatial variability. This is the first experimental study to specifically study these largerscale patterns with sufficient spatial resolution to systematically investigate how landscape and local characteristics affect the spatial variability in LGD. We measured vertical temperature profiles around a 0.49 km 2 lake in northeastern Germany with a needle thermistor, which has the advantage of allowing for rapid (manual) measurements and thus, when used in a survey, high spatial coverage and resolution. Groundwater inflow rates were then estimated using the heat transport equation. These near-shore temperature profiles were complemented with sediment temperature measurements with a fibre-optic cable along six transects from shoreline to shoreline and radon measurements of lake water samples to qualitatively identify LGD patterns in the offshore part of the lake. As the hydrogeology of the catchment is sufficiently homogeneous (sandy sediments of a glacial outwash plain; no bedrock control) to avoid patterns being dominated by geological discontinuities, we were able to test the common assumptions that spatial patterns of LGD are mainly controlled by sediment characteristics and the groundwater flow field. We also tested the assumption that topographic gradients can be used as a proxy for gradients of the groundwater flow field. Thanks to the extensive data set, these tests could be carried out in a nested design, considering both small-and large-scale variability in LGD. We found that LGD was concentrated in the near-shore area, but alongshore variability was high, with specific regions of higher rates and higher spatial variability. Median inflow rates were 44 L m −2 d −1 with maximum rates in certain locations going up to 169 L m −2 d −1 . Offshore LGD was negligible except for two local hotspots on steep steps in the lake bed topography. Large-scale groundwater inflow patterns were correlated with topography and the groundwater flow field, whereas small-scale patterns correlated with grain size distributions of the lake sediment. These findings confirm results and assumptions of theoretical and modelling studies more systematically than was previously possible with coarser sampling designs. However, we also found that a significant fraction of the variance in LGD could not be explained by these controls alone and that additional processes need to be considered. While regression models using these controls as explanatory variables had limited power to predict LGD rates, the results nevertheless encourage the use of topographic indices and sediment heterogeneity as an aid for targeted campaigns in future studies of groundwater discharge to lakes.

Evolution and dynamics of the vertical temperature profile in an oligotrophic lake

“…There are several studies reporting observed highfrequency (frequencies above 0.1 cycle min −1 , i.e., periods below ≈ 10 min) oscillations in thermocline regions of stratified lakes. Thorpe et al (1996) discussed internal waves with periods of between 6 and 10 min which they observed in Lake Geneva within a few hours after the passing of a disturbance that caused a rapid jump in the thermocline depth. The authors proposed three possible sources of these oscillations: (1) a soliton packet following the jump, (2) waves generated as the jump passed over or around rough topography, and (3) a moving disturbance produced by a thermocline jump near the shoreline accompanied by a region of high shear, low Richardson number, and a wake-like pattern of radiating internal waves.…”

Reply on RC1

“…Meinikmann et al (2015) observed that both the intensity of the contamination and its proximity to the lake inhibit nutrient retention within vadose zone and aquifer and allow significant phosphorus loads to be discharged into the lake. Tecklenburg and Blume (2017) found that large-scale LGD patterns were correlated with topography and groundwater flow field, whereas small-scale patterns correlated with grain size distributions of the lake sediment. However, the influence of geological conditions on spatial differences in LGD and associated nutrients input have not been well understood.…”

Reply on RC2

“…LGD and associated nutrients input are often characterized by large spatial variability due to the heterogeneity of geomorphy, surface hydrology, lakebed sediment lithology, hydrogeology, nutrient concentration levels of groundwater around lakes, etc (Wallace et al, 2020;Tecklenburg and Blume, 2017;Hare et al, 2017;Rosenberry et al, 2015;Meinikmann et al, 2015Meinikmann et al, , 2013Blume et al, 2013;Schneider et al, 2005). For example, Schmidt et al (2010) found that LGD rates were related to the drainage area/lake area ratio which promotes greater surface connectivity.…”

“…Although various methods of quantifying LGD exist (Tecklenburg and Blume;Dimova et al, 2015;Lewandowski et al, 2013;Stets et al, 2010), radioactive 222 Rn tracing has been widely and effectively used in recent years (Wallace et al, 2021(Wallace et al, , 2020Dabrowski et al, 2020;Hare et al, 2017;Petermann et al, 2018;Liao et al, 2018;Luo et al, 2018Luo et al, , 2016Luo et al, , 2014Burnett et al, 2017;Dimova et al, 2015Dimova et al, , 2013Gilfedder et al, 2015;Peterson et al, 2010;Schmidt et al, 2010Schmidt et al, , 2009. 222 Rn is a subset of 226 Ra and is widely present in inland rocks and water bodies.…”