Groundwater Sustainability in a Humid Climate: Groundwater Pumping, Groundwater Consumption and Land Use Change

Recent concerns in Wisconsin over groundwater quality and water supply have drawn attention to water use and groundwater withdrawals. We compiled groundwater pumping data for two Wisconsin counties to assess its reliability for use in numerical simulations of regional groundwater flow and to evaluate relations between development and population growth on demand for groundwater resources.

We estimated that in 2003, municipal wells supplied 60% (28.3 Mgal/d, or million gallons/day) of the total groundwater pumped (47.2 Mgal/d) in Waukesha County. In Sauk County, municipal systems supplied 25% (7.4 Mgal/day) of the groundwater used (29.6 Mgal/day). Sauk County has a much higher per capita water use rate than Waukesha County (500 and 107 gallons/person/day, respectively). Pumping for agricultural irrigation has a disproportionate effect on Sauk County water-use statistics, approximately 12,470 acres in Sauk County were in irrigated agricultural production in 2002 compared to 769 acres in Waukesha County. The population of Waukesha County is about six times greater than Sauk County, but total water use is only 60% higher.

Sauk County has a relatively low population density and water-intensive industrial, agricultural, and commercial activities. In Waukesha County, rapid population growth resulted in high residential water use, but this has been offset by decreases in water used by industry and agricultural irrigation. In Waukesha County, 45% of all groundwater pumped is for residential use; 39% for commercial and industrial use; 2% for irrigated agriculture and 2% for golf course irrigation. In Sauk County, about 12% of groundwater pumped is for residential use; 45% for commercial and industrial use; 39% for irrigated agriculture and less than 1% for golf course irrigation. Residential use averages about 74 gallons/person/day in Waukesha County and 61 gallons/person/day in Sauk County.

Impacts of water use on the hydrologic system are quite apparent in Waukesha County, where there are several hundred feet of drawdown in the potentiometric surface of the confined aquifer. Regional-scale impacts of groundwater pumping are not apparent in Sauk County. These conditions illustrate a counter-intuitive consequence of land use change: increasing residential development and population growth accompanied by a reduction in irrigated agriculture results in a significantly lower per capita water use rate in Waukesha County, but groundwater withdrawals are concentrated within the geographic region of water utility service areas. In contrast, effects of a much higher per capita water use rate in Sauk County are spread out over a large area; agricultural land overlies an unconfined sand and gravel aquifer and cumulative impacts of pumping are not readily apparent at a regional scale.

Use of the southeast Wisconsin regional flow model demonstrated that selected measures of hydrologic impact in the deep groundwater flow system were not sensitive to increases in pumping forecast for 2035. Both the extent of the 150-ft drawdown cone and the location of the regional groundwater divide were relatively insensitive to the tested rates of pumping. In this hydrogeologic setting, these measures are controlled by the geometry of the regional aquitard rather than the magnitude of pumping. Identifying areas appropriate for groundwater management might be better based on a measure of groundwater sustainability that is sensitive to pumping, such as baseflow to surface water features.

Project findings have several practical implications for current efforts to track water use in Wisconsin. Pumping from municipal systems is metered and reported, and these records are readily obtained. As a result, records of water use for Waukesha County, where a high percentage of pumping is from public water utilities, are better than for Sauk County, where more groundwater is pumped from private wells. Self-supplied pumping is not reported in Wisconsin. Based on our survey of high capacity well owners, well records incorrectly described the number of wells in use in 43% of the test cases (28 of 65). Additional survey and data collection efforts should focus on the number of wells in use by the largest-volume high capacity pumping permit holders. The potential for linking well permits and groundwater pumping records to locations and volumes of waste water discharge should also be examined.

Estimating groundwater use for golf course and agricultural irrigation may be preferable to overcoming potential resistance to, and inherent inaccuracies in, self-reporting for these categories. The Wisconsin Agricultural Statistics Service and the Applied Population Lab at the University of Wisconsin – Madison have expertise, and may provide efficient collaboration, in estimating pumping for these uses.