ABSTRACT

The potential for high groundwater levels to cause wet basements (groundwater flooding) is of concern to residents of communities in northwestern Indiana. Changes in recharge from precipitation increases during 2006–9, water-level changes from restoration of nearby wetlands in the Great Marsh in 1998–2002, and changes in recharge due to the end of groundwater withdrawals for water supply since 2005 in a community at Beverly Shores, Ind., were suspected as factors in increased groundwater levels in an unconfined surficial aquifer beneath nearby parts of a dune-beach complex. Results of this study indicate that increased recharge from precipitation and snowmelt was the principal cause of raised water levels in the dune-beach complex from 2006 to 2009.

Annual precipitation totals in 2006–9 ranged from 43.88 to 55.75 inches per year (in/yr) and were substantially greater than the median 1952–2009 precipitation of 36.35 in/yr. Recharge to groundwater from precipitation in 2006–9 ranged from 13.5 to 22 in/yr; it was higher than the typical 11 in/yr because of large precipitation events and precipitation amounts received during non-growing-season months. An estimated increase in net recharge from reduced groundwater use in Beverly Shores since 2005 ranged from 1.6 in/yr in 2006 to 1.9 in/yr in 2009.

Surface-water levels in the wetland were as much as about 1.1 feet higher in 2007–9 (after the 1998–2002 wetland restoration) than during seasonally wet periods in 1979–89. Similar surface-water levels and ponded water were likely during winter and spring wet periods before and after wetland restoration. High water levels similar to those in 2009 were measured elsewhere in the dune-beach complex near a natural wetland during the spring months in 1991 and 1993 after receipt of near record precipitation. Recharge from similarly high precipitation amounts in 2008–9 was also a likely cause of high groundwater levels in other parts of the dune-beach complex, such as at Beverly Shores.

Perennial mounding of the water table in the surficial aquifer indicates that the recharge that created the water-table mound originates within the dune-beach complex and not through flow from the adjacent hydrologic boundaries: the restored wetland, Lake Michigan, and Derby Ditch. Infiltrating precipitation causes most seasonal and episodic rises in groundwater levels beneath the dune-beach complex. Groundwater-level fluctuations lasting days to weeks in the dune-beach complex in 2008–9 were superimposed on a seasonal high water-table altitude that began with the recharge from snowmelt and rain in February 2009 and maintained through July 2009. Increases in water-table-mound altitude under the dune-beach complex recurred in 2008–9 in response to the largest rain events of 1 inch or more and to snowmelt. Smaller, shorter-term rises in water level after individual rain events persisted over hours to less than 1 week. Groundwater-level fluctuations varied over a relatively narrow range of about 2 to 3 feet, with no net fluctuations greater than 4 feet. Groundwater levels in or near low parts of the dune-beach complex were frequently within 0 to 6 feet of the land surface and indicate the potential for groundwater flooding.

Groundwater-level gradients from the water-table mound to wells next to surface-water discharges increase after rainfall and snowmelt events and recede slowly as groundwater discharges from the aquifer. Evapotranspiration is responsible for part of the general pattern of decreasing water-table altitudes observed from May to August 2009.

Rapid water-level rises in the restored wetland after precipitation do not likely have an effect on groundwater flooding elsewhere in the dune-beach complex. Surface-water-level fluctuations during this study generally varied over a narrower range, approximately from 1 to 1.5 feet, as compared with groundwater fluctuations, except after a very large, 10.77-inch rainfall. Time-delayed and smaller groundwater-level rises in wells near the restored wetland indicate a hydraulic delaying effect from the lower hydraulic conductivity of organic deposits in the subsurface near the marsh.

Results of a simplified, steady-state cross-sectional model of groundwater flow also indicate that increased recharge from precipitation and snowmelt was the principal cause of raised water levels in the dune-beach complex from 2006 to 2009. Rises in the simulated water-table crest caused by increased recharge from precipitation in 2006–9 ranged from about 2 to 4 feet. A simulated addition of 1.9 in/yr of recharge from the water supply change raised the crest of the water-table mound by about 0.7 foot at about 900 feet from the restored wetland. The simulated groundwater-level change from a wetland water-level increase was generally smaller than that caused by precipitation and water-supply-derived recharge. The effect of a 1.1 foot simulated increase in water level in the restored marsh diminished to about a 0.75 foot groundwater level increase at about 900 feet from the marsh and to about a 0.55 foot groundwater level increase at about 1,500 feet from the marsh. Actual groundwater-level changes from wetland water-level increases would be smaller than simulated values because the organic sediments separating the wetland and the surficial aquifer tend to delay the response of groundwater levels to recharge and surface-water-level changes.

First posted July 2011