Groundwater is an important source of water for agriculture, public supply and other uses. But, how much is too much to pump?
As a first step towards determining sustainable levels of groundwater usage, researchers developed a computer-based model to assess the availability of near-surface groundwater in each of the major watersheds in the Great Lakes region. The model incorporates both ecological and hydrogeological factors in the assessment.
The major ecological factor in the model is summer baseflow. Baseflow is the seepage of groundwater into streams that occurs when the water table is higher than the streambed. During most of the year, stream flow is composed of both baseflow (from groundwater) and runoff from the land due to rain or snow events. However, in the summer, when precipitation can be minimal, baseflow is key to maintaining a critical depth of water running through streams. Groundwater pumping by a single user or a group of can reduce stream flow and negatively impact the stream and associated ecosystem. Without adequate flowing water, fish may not survive or reproduce and many other important ecosystem functions cease. Water can be removed from some streams without ecological impact, but the amount of water that can be removed without negative impact is site specific. In this study, the ecological capacity of a groundwater aquifer was calculated as the maximum amount of groundwater pumping that could be sustained before a critical ecological flow limit was reached.
In some states in the Great Lakes basin, ecological flow limits have been defined as a function of stream type, i.e. temperature and size. However, since there is no single classification system for defining ecological flows across the basin, we use a uniform, maximum allowable steam depletion fraction for the entire study area. In the work reported here, we use a limit of 20 percent reduction in the seasonal low flow in a given stream segment, based on recommendations of Richter et al. (2012). The low flow, orQ90, is defined as the daily flow that is exceeded 90 percent of the time in the historical record for the stream segment. In other words, only 10 percent of the daily flows are below theQ90. In most cases, theQ90 corresponds to low flows during the summer months when baseflow dominates. Thus, the ecological flow limit thus corresponds to the 0.2Q90. In the publication corresponding to the work reported here, we also calculated ecological capacity of groundwater aquifers based on a more restrictive limit of 0.1Q90.
Hydrogeology is the study of the distribution and movement of groundwater in the soil and rocks of the Earth’s crust (commonly in aquifers). Figure 1 shows general flow times for various pathways through a typical system.
The major hydrogeological factor in the computer model is groundwater yield to a pumping well. For this study, groundwater yield constrained by hydrogeology is defined as the pumping rate at a specific time where drawdown at the well is equal to half of the original water table height. This metric is a rule of thumb that is used to indicate the point at which the physical capacity of the aquifer is vulnerable to over-pumping.
The maximum allowable pumping rate limited by ecologic or hydrogeological factors was calculated as follows. First, a hypothetical location for the pumping well was generated randomly in the U.S. portion of the Great Lakes basin. Next, the maximum allowable pumping rate limited by ecological factors was estimated using local aquifer information, such as aquifer hydraulic conductivity and thickness; distance between the well and the nearest steam segments; and the hydraulic connection between the stream segment and pumping well. The maximum allowable pumping rate limited by hydrogeological factors also was calculated, based on aquifer information similar to that used in the ecological limit calculations. The overall, maximum allowable pumping rate was taken as the lowest of the two calculated values (ecological vs. hydrogeological). This metric, the maximum available pumping rate, was calculated under hypothetical pumping scenarios at more than 600,000 locations in the US portion of the Great Lakes basin, effectively probing the possible maximum pumping rates over the full range of aquifer and stream conditions over the study area. Finally, the local, maximum pumping rates were averaged at the major (HUC 8) watershed level.
Figure 2 displays which factors (ecological or hydrogeological) were more influential in determining the availability of groundwater within a major watershed. The watersheds highlighted in dark blue are those that are heavily influenced by ecological factors, where in contrast, watersheds in yellow are highly influenced by hydrogeologic factors. The study results show that the importance of these constraints varies geographically. In most cases, the hydrogeological limits dominate where the aquifer hydraulic conductivities or thicknesses are relatively low, whereas the ecological limits are more prevalent where these aquifer values are high, streamflows are low, or stream networks are dense.
Groundwater availability for each of the major watersheds in the Great Lakes basin are shown in Figure 3. While the results of this study are not appropriate for determining local estimates of available groundwater, they present a relative basis for comparison of groundwater availability. Dark blues indicate groundwater rich watersheds, whereas the reds indicate watersheds with a groundwater supply that is more vulnerable to ecological impacts from excessive groundwater pumping or to hydrogeological limitations. The dark blue watersheds have 10 times more available groundwater than the watersheds shaded in light green, and about 100 times more available groundwater than the watersheds highlighted in red.
The results of this study, especially the variability observed across the Great Lakes basin, have important implications for water resources management and governance. These maps show where there may be greater capacity to support water intensive industries such as a brewery or a cannery. The maps may also help communities understand how sensitive their groundwater supply is to use and plan appropriately for future land use development. Additionally, they may inform water use planning and permitting decisions made by state agencies.
Potential Future Studies
The researchers suggested the following studies as logical and valuable extensions of this work:
- Model the cumulative impacts and the seasonal affects of multiple groundwater users on the flows downstream.
Future research should examine the collective (or cumulative) impacts of multiple groundwater users on a stream, and in particular how these impacts manifest during the various seasons. Analyzing these impacts under various scenarios and within the context of environmental flow restrictions may lead to important insights for water management studies.
- Explore the consequences of setting non-uniform environmental limits for streamflow depletion based upon stream type.
Certain stream types are known to be more sensitive to alterations in streamflow regimes and in particular changes to the amount of groundwater that contributes to streamflow. The Michigan Water Withdrawal Assessment Tool, for example, incorporates environmental constraints by stream type. Future research that extends this environmental stream classification over the entire U.S. portion of the Great Lakes basin may support analyses to evaluate the ecological impacts of proposed large water withdrawal proposals.
Richter, B. D., Davis, M. M., Apse, C., & Konrad, C. (2012). A presumptive standard for environmental flow protection. River Research and Applications, 28(8), 1312-1321.
Watson, K. A., Mayer, A. S., & Reeves, H. W. (2014). Groundwater availability as constrained by hydrogeology and environmental flows. Groundwater, 52(2), 225-238.