Meltwater Recharge Research Articles

Spatially heterogeneous discharge of glacial meltwater to drainages surrounding the ablating Coropuna ice cap, Peruvian Andes

ABSTRACT As tropical glaciers recede in response to global warming, glaciated watershed resilience is threatened by the loss of these important water reservoirs. In the Cordillera Occidental, Peru, we investigate the importance of glacial meltwater to mountain groundwater recharge and subsequent discharge to streams surrounding the rapidly retreating Coropuna ice cap. Using stable isotopes, solute concentrations, and radiocarbon activity, our results suggest that in remote ungauged systems, glacial input can be determined via geochemical analysis and a simple mixing model approach. We find that the proportion of glacial meltwater contribution to groundwater and surface waters is spatially heterogeneous. Differences between drainages are attributed to the degree of melting of headwater glaciers and the routing of waters through volcanic strata. These results provide evidence that glacial meltwater recharge is highly variable even within the same mountain block. Furthermore, the effects on streamflow will be disproportionate as the watersheds approach an ice-free state.

Analysis of runoff variations in an arid catchment based on multi-model ensemble- a case study in the Tarim River Basin in Central Asia

Runoff variation is of significant importance to the current and future water availability of a region, particularly in arid regions, and plays a crucial role in economic and social development. The Tarim River Basin, spanning an area of approximately 102 × 104 km2, is the largest inland river basin in China. Due to the basin’s extremely dry climate, water shortage is the most critical natural factor restricting socio-economic development in the region. This study focuses on analyzing the historical and future runoff changes of the four headstreams (the Kaidu, Aksu, Yarkand, and Hotan rivers) in the Tarim River Basin with historical observations and multiple-model projections. The results indicate that the runoff of the Tarim’s four headstreams showed an increasing trend during 1957–2022, with a remarkable increment of 40.70 × 108 m3, or 18% in percentage. Rising temperatures and precipitation are the main reasons for the runoff’s increase. Higher temperature accelerates the melting of glaciers, leading to enhanced recharge of meltwater, while more precipitation also boosts the increase in river runoff. Based on the modelling results from the extended SWAT (Soil and Water Assessment Tool), the runoff in the Kaidu, Aksu, Yarkand, and Hotan rivers will remain at a high level in the near future (−2035), with an average increase of 3.2%–7.55%. In the mid 21st century (2036–2065), the runoff of the Yarkand and Hotan River originated from the Kunlun and Karakoram mountains is expected to continue increasing by around 6.25%–15.2%. Under SSP126, the tipping point of glacier melt runoff in the basin may happen by 2058, while under SSP370 and SSP585 scenarios, it may be around 2080. The timing of peak water aligns with projections in the mountainous Asia, but is later compared to the tropical Andes, Western Canada, and the Swiss Alps, whose peak water has already been reached. The results of this study can provide a scientific basis for the allocation and efficient utilization of water resources in the Tarim River Basin and offer valuable insights into the forthcoming runoff changes in mountainous regions.

Hydrologic modeling of a perennial firn aquifer in southeast Greenland

AbstractA conceptual model, based on field observations and assumed physics of a perennial firn aquifer near Helheim Glacier (southeast Greenland), is evaluated via steady-state 2-D simulation of liquid water flow and energy transport with phase change. The simulation approach allows natural representation of flow and energy advection and conduction that occur in vertical meltwater recharge through the unsaturated zone and in lateral flow within the saturated aquifer. Agreement between measured and simulated aquifer geometry, temperature, and recharge and discharge rates confirms that the conceptual field-data-based description of the aquifer is consistent with the primary physical processes of groundwater flow, energy transport and phase change. Factors that are found to control simulated aquifer configuration include surface temperature, meltwater recharge rate, residual total-water saturation and capillary fringe thickness. Simulation analyses indicate that the size of perennial firn aquifers depends primarily on recharge rates from surface snowmelt. Results also imply that the recent aquifer expansion, likely due to a warming climate, may eventually produce lakes on the ice-sheet surface that would affect the surface energy balance.

Spatial variability of runoff recharge sources and influence mechanisms in an arid mountain flow‐producing zone

AbstractIn‐depth knowledge of the hydrological conditions of basins in arid regions can help to maintain the long‐term water security. The mechanisms of runoff generation and flow paths in different landscape areas are not fully known due to spatial variability. We have explored the mechanisms by which spatial variability affects the runoff recharge sources in the Xiying River Basin. The results showed that the spatial variability of the sources and generation mechanisms of runoff in the arid mountain flow‐producing zone is the result of the combined effect of different landscape, elevation and meteorological elements. In the upstream area, the short mean transit time (MTT) combined with the high contribution of recharge from the meltwater (29.32%) and the proportion of pre‐event water (between 60% and 90%) means that runoff production processes are strongly controlled by meltwater recharge and precipitation events. The MTT and the proportion of pre‐event water in the midstream area are ranked in the middle of the three landscape areas, but the highest contribution of precipitation (16.57%) indicates the importance of precipitation to runoff production processes. In the downstream area, the MTT is the longest and the proportion of pre‐event water is the smallest, and groundwater remains the basis for long‐term stability of runoff (groundwater contribution rate: 40.62%). By enhancing the spatial resolution for identifying runoff recharge sources can reduce the model uncertainty by 69%. Thus, taking full account of the spatial heterogeneity in arid regions is an important initiative to achieve scientific water resources management.

Variability in Timing and Transport of Pleistocene Meltwater Recharge to Regional Aquifers

AbstractThe impacts of Pleistocene glaciation on groundwater flow systems in sedimentary basins are widely recognized, but the timing and distribution of subglacial recharge events remain poorly constrained. We investigate the spatial and temporal variability of recharge events from glaciations over the last 2 million years in the Williston Basin, Canada. Integration of fluid chemistry, stable isotope data, and transport modeling indicate that meltwater arrived at depths of ∼600–1000 m in the northcentral region of the Williston Basin at two separate time periods, 75–150 and 300 ka, which we attribute to permeability differences between stacked aquifer systems. Our findings indicate that meltwater recharge extended along the northern margin of the Williston Basin as well as previously identified recharge areas to the east. Given the distance of measurements from recharge areas, evidence of recharge from the early to mid‐Pleistocene appears to be preserved in the Williston Basin.

Analysis of the groundwater flow system in a high-altitude headwater region under rapid climate warming: Lhasa River Basin, Tibetan Plateau

Study regionLhasa River Basin (LRB) on the Tibetan Plateau, China. Study focusTibetan Plateau has been undergoing climate warming in the past five decades. The hydrological processes in the LRB, a representative alpine headwater region basin on the Tibetan Plateau, are changing in response to climate warming. However, characteristics of groundwater recharge and discharge and response of the groundwater flow system to future climate change in this region remain unclear. This study constructed a three-dimensional numerical model to simulate the groundwater flow variations under different future climate change scenarios. New hydrological insights for the regionApproximately 13.6 % (∼71.6 mm/yr) of annual precipitation recharges groundwater. Glacier meltwater recharge is 10 % of the total groundwater recharge across the entire LRB, and this proportion increases to 34.8 % in the Yangbajing subbasin. More than 80 % of the groundwater circulates within 0∼0.3 km below ground surface. The groundwater flow system is dominated by travel times between 10–100 years and flow path distances less than 10 km. The baseflow shows an increasing trend in response to future climate change, and the increasing trends range from 0.07 to 0.21 m3/s per year under different future climate change scenarios. The baseflow variation indicates that wet year will become wetter and dry year will become drier in the future.

Sensitivity of subglacial drainage to water supply distribution at the Kongsfjord basin, Svalbard

Abstract. By regulating the amount, the timing, and the location of meltwater supply to the glacier bed, supraglacial hydrology potentially exerts a major control on the evolution of the subglacial drainage system, which in turn modulates ice velocity. Yet the configuration of the supraglacial hydrological system has received only little attention in numerical models of subglacial hydrology so far. Here we apply the two-dimensional subglacial hydrology model GlaDS (Glacier Drainage System model) to a Svalbard glacier basin with the aim of investigating how the spatial distribution of meltwater recharge affects the characteristics of the basal drainage system. We design four experiments with various degrees of complexity in the way that meltwater is delivered to the subglacial drainage model. Our results show significant differences between experiments in the early summer transition from distributed to channelized drainage, with discrete recharge at moulins favouring channelization at higher elevations and driving overall lower water pressures. Otherwise, we find that water input configuration only poorly influences subglacial hydrology, which instead is controlled primarily by subglacial topography. All experiments fail to develop channels of sufficient efficiency to substantially reduce summertime water pressures, which we attribute to small surface gradients and short melt seasons. The findings of our study are potentially applicable to most Svalbard tidewater glaciers with similar topography and low meltwater recharge. The absence of efficient channelization implies that the dynamics of tidewater glaciers in the Svalbard archipelago may be sensitive to future long-term trends in meltwater supply.

Holocene Lake Evolution and Glacial Fluctuations Indicated by Carbonate Minerals and Their Isotopic Compositions in the Sediments of a Glacial Melt Recharge Lake on the Northwestern Tibetan Plateau

Lakes and glaciers are widely distributed on the Tibetan Plateau and are linked via hydrological processes. They are experiencing rapid changes due to global warming, but their relationships during the Holocene are less well known due to limited coupled geological records. Here, we analyzed the δ13C-VPDB and δ18O-VPDB values and ion content of calcite and aragonite in a 407-cm-long sediment core from Guozha Co, a closed basin on the northwestern Tibetan Plateau supplied by glacial meltwater, in order to understand how the lake responded to glacier changes during the Holocene. Our results indicate that the glacial meltwater lowered the lake’s temperature and the δ18Olake water and δ18Oendogenic + authigenic carbonate values and diluted the ion concentrations in the lake water. Three stages of evolution, 8.7–4.0, 4.0–1.5, and 1.5 kyr BP to present, are distinguished based on the decrease in glacial meltwater recharge. Guozha Co has been a closed basin since at least 8.7 kyr BP, and it has changed from a fresh water lake during 8.7–1.5 kyr BP to a brackish lake from 1.5 kyr BP to present due to several climate events. The famous 4.2 kyr BP cold event was identified in the core at 4.0 kyr BP, while warm events occurred at 6.2, 3.9, 2.2, 0.9, and 0.4 kyr BP. Both glaciers and lakes in this area are controlled by climate, but they exhibit opposite changes, that is, glaciers retreat and lakes expand, and vice versa. Our results provide an accurate interpretation of the cold events based on carbonate minerals and carbon–oxygen isotopes in glacial meltwater–recharged lake sediments.

Assessment of paleo-recharge under the Fennoscandian Ice Sheet and its impact on regional groundwater flow in the northern Baltic Artesian Basin using a numerical model

The study investigates the mechanism of glacial meltwater recharge under the Fennosciandian Ice Sheet during the last glacial maximum (LGM) and its impact on regional groundwater flow in the northern Baltic Artesian Basin (BAB) in Estonia and Latvia. The current hypothesis is that a flow reversal occurred in the BAB due to subglacial recharge during the LGM. This hypothesis is supported by an extensive dataset of geochemical and isotopic measurements in the groundwater of northern Estonia, exhibiting significant depletion in δ18O with respect to modern precipitation. To verify the consistency of this hypothesis and better understand groundwater flow dynamics during the LGM period, a numerical model is developed for this area. Two cross-sectional models have been created across the northern BAB, in which groundwater flow and the transport of δ18O have been simulated from the beginning of the LGM to present-day. Several simulations were performed with different subglacial boundary conditions, to investigate the uncertainty related to subglacial recharge of meltwater during the LGM and the subsequent flow reversal in the northern BAB. Several simulations provide a satisfying fit between computed and observed values of δ18O, which means that the hypothesis of subglacial recharge of meltwater is consistent with δ18O distribution. The numerical model suggests that preservation of meltwater in northern Estonia is controlled by confining layers and the proximity to the outcrop area of aquifers, located in the Gulf of Finland. The results also suggest that glacial meltwater has been preserved under the Baltic Sea in the Gulf of Riga.

对欧洲深部含水层古地下水补给条件的认识

Climatic instability during the late Pleistocene has been reflected in the pattern of groundwater recharge. This report summarizes palaeoclimate knowledge during the late Weichselian in Europe. During this period the majority of northern Europe was covered by thick ice sheets and permafrost, preventing aquifers from recharging. In contrast, southern Europe was generally free of these palaeoclimatic features. Palaeoclimatic information has been combined with isotope data to better understand the palaeorecharge conditions and recharge timing across the European continent. The 18O and 2H relationship shows latitudinal plus climatic influences. Radiocarbon data show that while southern European aquifers have generally been recharged continuously during the last 40,000 years, northern European aquifers typically show a recharge gap during the Last Glacial Maximum. Areas that underwent continuous recharge during the entire late Pleistocene period can also be distinguished from areas where recharge to aquifers was prevented during the Last Glacial Maximum. Finally, several examples are presented of melt-water recharge or subglacial recharge. The identification of such diversity in the groundwater palaeorecharge in Europe is of great importance for modellers developing management schemes for groundwater resources.

Isotopically‐depleted late Pleistocene groundwater in Columbia River Basalt aquifers: Evidence for recharge of glacial Lake Missoula floodwaters?

Late Pleistocene outburst flooding of ice‐dammed glacial Lake Missoula, and possible discharge from the Cordilleran Ice Sheet (CIS), catastrophically altered the northwestern United States landscape, yet little is known about potential infiltration of flood waters into the subsurface. This study provides compelling evidence for the presence of late Pleistocene CIS‐related recharge waters in the Columbia River Basalt Aquifers (CRBAs) in central Washington. CRBA groundwaters with corrected 14C ages from 15.7 and 33.3 k yrs BP (during periods of flood events) have anomalously low δ18O values (−18.9 to −17.6‰), compared to late Pleistocene soil waters (−16.1 to −13.4‰) and modern precipitation in the region (average −15.9‰), consistent with CIS‐related meltwater recharge. These results have implications for our understanding of megaflood phenomena on earth and Mars.

Drainage beneath ice sheets: groundwater–channel coupling, and the origin of esker systems from former ice sheets

The nature of the drainage system beneath ice sheets is crucial to their dynamic behaviour but remains problematic. An experimentally based theory of coupling between groundwater and major channel systems is applied to the esker systems in the area occupied the last ice sheet in Europe, which we regard as a fossil imprint of major longitudinal drainage channels. We conclude that the large-scale distribution and spacing of major eskers is consistent with the theory of groundwater control, in which esker spacing is partly controlled by the transmissivity of the bed. It is concluded that esker patterns reflect the large-scale organisation of the subglacial drainage pattern in which channel development is coupled to groundwater flow and to the ice sheet's dynamic regime. The theory is then used to deduce: basal meltwater recharge rates and their spatial variability from esker spacing in an area in which the ice sheet was actively streaming during its final retreat; patterns of palaeo-groundwater flow and head distribution; and the seasonally varying magnitude of discharge from stream tunnels at the retreating ice sheet margin. Major channel/esker systems appear to have been stable at least over several hundred of years during the retreat of the ice sheet, although major dynamic events are demonstrably associated with major shifts in the hydraulic regime. Modelling suggests: that glaciation can stimulate deep groundwater circulation cells that are spatially linked to channel locations, with groundwater flow predominantly transverse to ice flow; that the circulation pattern has the potential to create large-scale anomalies in groundwater chemistry; and that the spacing of channels will change through the glacial cycle, influencing water pressures in stream tunnels, subglacial hydraulic gradients and effective pressure. If the latter is reduced sufficiently, it could trigger enhanced bed deformation, thus coupling drainage to ice sheet movement. It suggests the possibility of distinctive phases of sediment deformation and drumlin mobilisation during a glacial cycle.

Groundwater and climate dynamics derived from noble gas, 14C, and stable isotope data

Research Article| May 01, 2008 Groundwater and climate dynamics derived from noble gas, 14C, and stable isotope data Stephan Klump; Stephan Klump 1Eawag, Swiss Federal Institute of Aquatic Science and Technology, 8600 Duebendorf, Switzerland and Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, 8092 Zurich, Switzerland Present address: EBA Engineering Consultants, Ltd., Calcite Business Centre, Unit 6, 151 Industrial Road, Whitehorse, Yukon Y1A 2V3, Canada; E-mailsklump@eba.ca. Search for other works by this author on: GSW Google Scholar Tim Grundl; Tim Grundl 2Department of Geosciences, University of Wisconsin–Milwaukee, Milwaukee, Wisconsin 53201, USA Search for other works by this author on: GSW Google Scholar Roland Purtschert; Roland Purtschert 3Physics Institute, University of Berne, 3012 Berne, Switzerland Search for other works by this author on: GSW Google Scholar Rolf Kipfer Rolf Kipfer 4Eawag, Swiss Federal Institute of Aquatic Science and Technology, 8600 Duebendorf, Switzerland and Institute of Isotope Geochemistry and Mineral Resources, ETH Zurich, 8092 Zurich, Switzerland Search for other works by this author on: GSW Google Scholar Author and Article Information Stephan Klump Present address: EBA Engineering Consultants, Ltd., Calcite Business Centre, Unit 6, 151 Industrial Road, Whitehorse, Yukon Y1A 2V3, Canada; E-mailsklump@eba.ca. 1Eawag, Swiss Federal Institute of Aquatic Science and Technology, 8600 Duebendorf, Switzerland and Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, 8092 Zurich, Switzerland Tim Grundl 2Department of Geosciences, University of Wisconsin–Milwaukee, Milwaukee, Wisconsin 53201, USA Roland Purtschert 3Physics Institute, University of Berne, 3012 Berne, Switzerland Rolf Kipfer 4Eawag, Swiss Federal Institute of Aquatic Science and Technology, 8600 Duebendorf, Switzerland and Institute of Isotope Geochemistry and Mineral Resources, ETH Zurich, 8092 Zurich, Switzerland Publisher: Geological Society of America Received: 13 Nov 2007 Revision Received: 24 Jan 2008 Accepted: 25 Jan 2008 First Online: 02 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2008 Geological Society of America Geology (2008) 36 (5): 395–398. https://doi.org/10.1130/G24604A.1 Article history Received: 13 Nov 2007 Revision Received: 24 Jan 2008 Accepted: 25 Jan 2008 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Stephan Klump, Tim Grundl, Roland Purtschert, Rolf Kipfer; Groundwater and climate dynamics derived from noble gas, 14C, and stable isotope data. Geology 2008;; 36 (5): 395–398. doi: https://doi.org/10.1130/G24604A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Noble gas data in conjunction with stable isotopes and 14C ages of groundwater samples from southeastern Wisconsin, USA, indicate a soil cooling of at least 6.5–7 °C during the last glacial period compared with modern soil temperatures. Because stable isotope and excess Ne data indicate that none of the samples contains any significant portions of glacial meltwater, samples with 14C ages between 12 and 26 ka B.P., which is the time when the study area was ice covered, most likely infiltrated during short periods of ice retreat or represent recharge containing a significant proportion of precipitation rather than subglacial meltwater recharge. Further, all samples except for those recharged before the last glacial period show a strong correlation between noble gas temperature and δ18O. By contrast, δ18O values of samples older than ca. 28 ka B.P. are too heavy with respect to their noble gas temperatures. This might be due to a stronger influence of an isotopically enriched moisture source from the Gulf of Mexico. The amount of excess air, which is closely linked to the magnitude of groundwater table fluctuations, increases shortly before and at the beginning of the last glacial period, suggesting that recharge dynamics changed considerably during that time period. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The role of meltwater in glacial processes

Water plays a dominant role in many glacial processes and the erosional, depositional and climatic significance of meltwaters and associated fluvioglacial processes cannot be overemphasized. At its maximum extent c. 20,000 years ago, the volume of the Laurentide ice sheet was 33 × 10 6 km 3 (about the same as the volume of all ice present today on planet Earth). The bulk of this was released as water in little more than 10,000 years. Pulses of meltwater flowing to the Atlantic Ocean from large ice dammed lakes altered thermohaline circulation of the world's oceans and global climate. One such discharge event via Hudson Bay at 8200 years BP released 160,000 km 3 of water in 12 months. Global sea levels recovered from glacial maximum low stands reached at about 20,000 years ago at an average rate of 15 m per thousand years but estimates of shorter term rates suggest as much as 20 m sea level rise in 1000 years and for short periods, rates as high as 4 m per hundred years. Meltwaters played a key role in lubricating ice sheet motion (and thus areal abrasion) across the inner portions of the ice sheet where it slid over rigid crystalline bedrock of the Canadian Shield. The recharge of meltwater into the ice sheets bed was instrumental in generating poorly sorted diamict sediments (till) by sliding-induced shearing and deformation of overpressured sediment and soft rock. The transformation of overpressured till into hyperconcentrated slurries in subglacial channels may have generated a highly effective erosional tool for selective overdeepening and sculpting of bedrock substrates. Some workers credit catastrophic subglacial ‘megafloods’ with the formation of drumlins and flutes on till surfaces. Subglacial melt river systems were instrumental in reworking large volumes of glaciclastic sediment to marine basins; it has been estimated that less than 6% of the total volume of glaciclastic sediment produced during the Pleistocene remains on land. Fluvioglacial and glaciolacustrine sediments and landforms dominate large tracts of the ‘glacial’ landscape in North America. The recharge of subglacial meltwater into underlying bedrock and sediment aquifers created transient reversals in the long-term equilibrium flow directions of basinal fluids. With regard to pre-Pleistocene glacial record, meltwaters moved enormous volumes of terrestrial ‘glaciclastic’ sediment to marine basins and thus played a key role in preserving a record of glaciation, a record otherwise almost entirely lost on land.

A conceptual, linear reservoir runoff model to investigate melt season changes in cirque glacier hydrology

This paper presents a conceptual, linear reservoir runoff model and applies it to a small glacierized cirque basin in the French Pyrénées over the 1995 melt season. A series of modelling experiments are undertaken: (i) to explore the response of diurnal hydrograph form to seasonal changes in surface meltwater recharge and glacier storage and routing processes and (ii) to investigate the possible structure of the hydrological system of this remnant glacier. High resolution, spatially- and temporally distributed observations of snow and ice-melt (and precipitation records) are used to estimate bulk meltwater inputs, which feed into a lumped meltwater drainage model. Empirical hydrograph recession limb analysis provides a basis to identify the most likely ‘structure’ of the glacier's hydrological system. This structure is then represented in the model by two (‘fast’ and ‘slow’) linear reservoirs. Although fast reservoir storage coefficients show only a moderate decline (13.00–5.25 h), the proportion of bulk meltwater entering this reservoir increases as the glacier snowline retreats and the slow reservoir storage coefficient decreases (45.00–17.75 h); consequently modelled hydrographs become increasingly peaked over the ablation season. Later in the melt season, the drainage system is mathematically best represented as a single reservoir (with a storage coefficient of 6.00–8.25 h) due to meltwater production occurring mainly in the lower-mid ablation zone, reduction in the extent (capacity) of the slower storage areas, and/or integration of the slow and fast pathways. In terms of glacier hydrology, the modelling experiments suggest that the fast reservoir represents ice-melt draining into a semi-distributed system beneath the lower glacier and the slow reservoir represents a snowpack-fed distributed system below the upper glacier. The nature of storage and routing within the hydrological system and the degree to which these processes are significant in determining outflow from this vestigial glacier compared with larger glaciers raises some interesting scale-related issues. The paper also demonstrates the utility of simple conceptual modelling approaches for investigating glacier hydrological systems, in addition to their more traditional application in runoff forecasting.

Isotopic evidence for glacial meltwater recharge to the Cambrian-Ordovician aquifer, north-central United States

The chemistry of water in the Cambrian-Ordovician aquifer in six midwestern states has been studied as part of the Northern Midwest Regional Aquifer-System Analysis of the U.S. Geological Survey. Dissolved-solids concentrations generally increase perpendicular to the direction of regional groundwater flow, from less than 400 mg/liter in southeast Minnesota, southwest Wisconsin, and northeast Iowa to more than 10,000 mg/liter in northwest Missouri. Isotopic ratios of hydrogen and oxygen are significantly depleted from north to south, with an areal distribution approximately parallel to the distribution of dissolved solids. For example, δ 18O in southern Iowa and northern Missouri is about 6 parts per thousand lighter than δ 18O of modern recharge water in Minnesota and Wisconsin. Covariance between δ 18O and δD of the groundwater, similar to that of modern precipitation, suggests that the differences in isotopic ratios between groundwater and modern recharge water reflect meteoric signatures of water during past recharge events rather than geochemical processes such as isotopic exchange with aquifer materials. The pronounced parallelism between the distribution of isotopes and dissolved solids over large areas probably reflects largescale recharge of Pleistocene glacial meltwater into the aquifer system, which probably had a paleoflow system with a gradient from northeast to southwest rather than from northwest to southeast.