HYDROLOGIC ASSESSMENT OF SEASONAL POND RESPONSE TO LARGE-VOLUME GROUNDWATER WITHDRAWAL

Seasonal ponds are small, temporary water bodies that provide important habitats for a variety of species. They typically fill with water in the spring and are dry by late summer, with periodic drying ensuring the absence of predatory fish. The length of time they retain water, known as the hydroperiod, is a critical factor in the survival rate of amphibian larvae. Precipitation, evapotranspiration, surface runoff, surface water inlets and outlets, and groundwater inflow and outflow contribute to the hydrologic budget of the seasonal pond resulting in natural hydroperiod variations from year to year. High volume extraction of groundwater in the vicinity of these ponds may however increase the incidence of short-hydroperiod years, thereby adversely impacting habitat suitability for species dependent on this ecosystem for reproduction. Temperature, chemical, and hydraulic head data were collected to characterize the seasonal pond hydrologic controls and identify existing and potential ground-water withdrawal impacts at 18 seasonal ponds in the Pawcatuck Watershed of southern Rhode Island, including 13 in close proximity to existing public supply wells and 5 control ponds. Climatalogic and control pond data were used to develop hydrologic response models for each wellfield pond. 9 of 13 wellfield ponds exhibited greater than expected water loss, indicating a probable impact from groundwater withdrawals. The hydroperiod was sufficiently shortened in 5 of these wellfield ponds to preclude successful reproduction of wood frogs and salamanders. Hydrologic data (water chemistry and water levels) reveal that all 9 of the impacted wells were identifiable as well-connected to the ground-water system, whereas 3 of the unaffected ponds were readily identified as perched. Hydrologic assessment of seasonal ponds prior to installation of large-volume wells can therefore be an effective approach to predict the potential impact of a proposed withdrawal.