GROUNDWATER POLLUTION FROM ONSITE DISPOSAL SYSTEMS AND OTHER LAND USES ON THE ‘EWA PLAIN, O’AHU

Abstract

Due to historical and current agricultural practices, the use of recycled wastewater, and high-density on-site disposal systems (OSDS), the ‘Ewa Plain is at risk for harmful impacts related to excess nutrients entering its groundwater and coastal environments. Groundwater pollution on the flat, low gradient ‘Ewa Plain can transport nutrient pollution from source to ocean on short timescales. Identifying the type and concentrations of nutrients present in groundwater is important in understanding the overall impact that these nutrient fluxes have on groundwaters and coastal ecosystem health. Being comprised of thick layers of limestone covering most of the region’s surficial and subsurface geology, the ‘Ewa Plain is a unique geologic setting in Hawaii, and shallow groundwater flow within its flat, low gradient limestone wedge can transport nutrient pollution from source to the ocean on short timescales. This study combines several different approaches to locate and identify relative contributions of groundwater pollution within these sedimentary rocks, including electrical resistivity tomography (ERT), coastal salinity surveys, and geochemical tracers to identify sources of excess nutrients. A numerical groundwater model was created within the MT3DMS modeling environment to incorporate these results and simulates the relative impacts of total dissolved nitrogen (TDN) and dissolved inorganic nitrogen (DIN) pollution within this region. Results from near shore in situ ERT transects and lab tests of sediments indicate a layer of unconsolidated and consolidated beach sediments sitting atop a deeper limestone unit completely saturated with water, but failed to locate karstic conduits at either of the locations. Two along shore salinity surveys conducted located several zones of salinity below 35, and all but 19 measured salinity points were below 31. Submarine groundwater discharge along the shore appears to be mainly saline and diffuse, and may be emanating further offshore. Measured beachface pore water samples and groundwater well samples showed elevated nutrient values compared to standard ocean water. The numerical groundwater model environment MT3DMS was used to simulate the relative impact of total dissolved nitrogen and dissolved inorganic nitrogen from five different sources: background soil processes, OSDS, agriculture, golf courses, and recycled wastewater irrigation (R-1 water). The estimated impact from the MT3DMS model of total dissolved nitrogen and dissolved inorganic nitrogen into the ‘Ewa Plain indicate that the sources of highest relative anthropogenic nitrogen loading to groundwaters are widespread R-1 irrigation and more localized high-density OSDS related effluent.