Hydrogeologic Characterization of a Karst Groundwater System in Lititz, Lancaster, County, Pennsylvania

Abstract

This study used various techniques to delineate groundwater flow types and the potential for fast groundwater flow in a karst system. By characterizing flow types the delineation of a wellhead protection source area was possible. Fluorescent dye tracer tests, water chemistry analyses, groundwater modeling, groundwater temperature monitoring, and geophysics were used to characterize factors which could affect groundwater flow (diffuse versus conduit flow) in a karst aquifer near Lititz, Lancaster County, Pennsylvania. Sinkholes, springs, and a well were selected for tracer test injections, water chemistry sampling, and temperature monitoring. Geophysical surveys were used to identify potential pathways (conduits) between tracer injection and sampling points at a selected site within the study area but no large conduits were found. Groundwater modeling, using the USGS software Modflow, examined how various types of karst conduit networks can lead to different sizes of source areas for wellhead protection. Two sites were studied in detail within the study area: Lititz Spring and the Buch well. Lititz Spring exhibited diffuse flow characteristics with relatively constant_ seasonal ion concentrations, diffuse tracer results, and relatively constant water levels following storm events. The Buch well site exhibited some conduit characteristics such as seasonal and time dependent characteristics with high variation in seasonal ion concentrations, high concentration of dye recovered, and rapid water level rise following storm events. The contrast in flow types at the two sites is seen in the coefficient of variation in hardness, which was 2 to 3 times higher in the Buch well than at Lititz spring. Groundwater residence times were calculated to be approximately 2 days and 9 days for the Buch well and Lititz spring respectively. Maximum dye concentrations found in tracer tests were <l µg/L for Lititz spring and estimated at >100 µg/L for the Buch well. Water levels increased by greater than 1 meter following storm events in the Buch well while water level variation at Lititz Spring was negligible (<0.25 m). The overall lack of springs and surface water features combined with field data support evidence of only localized conduit flow passages within the watershed. Groundwater modeling showed that a well-connected and expansive conduit system creates a larger wellhead source protection area as compared with only a localized karst conduit system. Tracer tests alone may not be the best way to delineate groundwater protection source areas in karst systems with a relatively localized karst conduit system. This study shows that characterization in karst systems needs to be site specific and that continuous monitoring of flow and geochemistry characteristics can be used in addition to tracer tests in delineating source protection areas.