Groundwater flux as a determinant of coastal-zone upland loss: a case study from the Pennsylvania coast of Lake Erie, USA

Climate change in the Great Lakes Basin of North America over the next several decades is projected to lead to significant changes to coastal environments. Groundwater-driven coastal bluff recession should increase in areas where groundwater forcing is important and lead to increased loss of coastal uplands. The latter is an issue in NW Pennsylvania because of coastal development pressures, and because the state ranks within the top five US states in grape production, most of which occurs within 5 km of the Lake Erie coastline. In 2007, viticulture contributed almost $2.4 billion to the state economy. An analysis of a 20-km stretch of coast shows that bluff retreat is pervasive and variable under current climatic conditions. Over a 9-year time frame, bluff change rates ranged from −4.2 to +0.98 m/year. In general, higher retreat rates (−0.2 to −0.65 m/year) occurred along the sandy central beach–ridge sector which lacks significant surface drainage. Lower retreat rates (−0.10 to −0.25 m/year) occurred along coastal sectors where surface drainage networks are well developed. Conservative estimates of groundwater discharge at the bluff correlate strongly (r = 0.74, p < 0.001) with bluff retreat rate. Groundwater is inferred to be the principal driving mechanism for both bluff retreat and spatial variability in retreat rates on this coast. Other common factors that may spatially influence bluff retreat elsewhere (bluff height, land use, beach width) do not correlate strongly with retreat rate.