Aquifer Vulnerability Evaluation Based on Water Table Fluctuations

Abstract:

Preferential flow paths within fractured rock unsaturated zones are pervasive and can lead to rapid episodic aquifer recharge. Episodic recharge can transport contaminants, pathogens, and other substances rapidly to the aquifer bypassing much of the matrix material that might otherwise mediate contamination through sorption and degradation processes. This critical recharge mechanism has not been effectively introduced into groundwater vulnerability assessments, yet it affects water quality in supply wells and can compromise aquatic ecosystems. This study aims to evaluate the vulnerability of shallow fractured rock aquifers in three watersheds in New York state by examining rapid episodic recharge through the unsaturated zone. The region is primarily glacial till of variable thickness underlain by fractured sandstone and carbonate rocks. For this purpose, we use the Episodic Master Recession (EMR) method to evaluate rapid recharge events using groundwater hydrographs and precipitation data. The EMR method provides means to evaluate long-term hydrographs to discern and illuminate trends with storm characteristics, seasons, soil conditions, and other factors. We evaluated 3 years of high-resolution water level and precipitation data for 27 wells where fluctuations are not influenced by pumping or other external forces. Using recharge estimates and storm characteristics from episodic events identified by the EMR programs, we considered circumstances of each episode including total precipitation duration, and intensity, and found that storm duration has the largest influence on recharge to precipitation ratios. This study indicates that, for this region, where recharge is occurring through preferential flow paths, long-duration precipitation events are more likely than short-duration events to have detrimental effects on water quality.