Exploring submarine groundwater discharge into the Delaware Inland Bays over diverse scales with direct measurements and modeling
Date
2012
Authors
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Publisher
University of Delaware
Abstract
Fresh submarine groundwater discharge (SGD) has been identified as an important source of nutrient fluxes to bays. One of the primary difficulties for quantifying fluxes of SGD-borne nutrients is that estimates of SGD vary widely and depend both on location and method of measurement. This study uses watershed-scale models and site-scale measurements to estimate net fresh SGD and characterize temporal and spatial controls on variability of SGD. MODFLOW models of the Delaware Inland Bays, a Mid-Atlantic coastal watershed, show that fresh SGD is 40% greater than stream baseflow on average—5% greater in March and 73% greater in August. Bays account for 12% of watershed area and bay arms reach nearly two-thirds the distance to the landward watershed edge. Flux to these arms accounts for the majority of SGD. Sensitivity analyses indicate that horizontal hydraulic conductivity in the aquifers is the dominant variable controlling the magnitude of fresh SGD, and that SGD is generally greater than baseflow across the range of reasonable parameter values. Hydraulic properties in shallow aquifers have greater potential impact on SGD rates than deeper aquifer properties. Particle tracking indicates that 40% of SGD is greater than one year and ranges upwards of 100,000 years old. A multi-disciplinary field investigation conducted at Holts Landing, Indian River Bay, DE, including 552 seepage meter and 92 porewater salinity measurements along with marine seismic and resistivity surveys, characterized shallow sediments, porewaters salinity and SGD. These data show a link between geologic heterogeneity and the spatial distribution of SGD on a range of spatial and temporal scales. This work finds that low-permeability paleovalley sediments at the bayfloor overlie underlying fresh groundwater plumes. SGD up to 100% fresh was measured along the interfluve coast; farther offshore fresh SGD was absent. Above the paleovalley, fresh SGD was absent but fresh groundwater flowed into the bay along the edge of the low-permeability sediment at the paleovalley/interfluve boundary, where SGD up to 36% fresh was measured. High saline SGD rates were also measured along the paleovalley/interfluve border. This suggests the presence of a shore-parallel density-driven circulation cell similar to that expected perpendicular to the interfluve coastline. In-situ porewater salinity was consistent with SGD salinity patterns. Clusters of seepage meter measurements (1m spacing) indicate that SGD varies more on spatial scales of 1 to 5 m than temporally on tidal timescales, which may explain the absence of tidal and seasonal temporal trends in the complete dataset. This study shows that groundwater flowpaths, mixing of fresh and saline groundwater, and the spatial distribution of SGD are controlled by local geology and are sensitive to temporal variations in the terrestrial hydraulic gradient. Modeling indicates that it will take years before efforts to reduce nutrient loading to land surface will be seen in reduced nutrient loading through SGD. It is important for managers to consider implications of these complications in estimates of SGD-born nutrient loads.