Submarine Groundwater Discharge and Stream Baseflow Sustain Pesticide and Nutrient Fluxes in Faga'alu Bay, American Samoa

Abstract

It is increasingly recognized that groundwater discharge in the form of stream baseflow and submarine groundwater discharge (SGD) plays an important role in contaminant transport. This study seeks to demonstrate the importance of groundwater flow for the distribution and transport of selected pesticides and nutrients in the Faga'alu aquifer on the island of Tutuila in American Samoa. Field measurements, including seepage runs and analysis of stream and groundwater for pesticides and nutrients, were combined with hydrological modeling. Selected analytes were glyphosate (GLY), dichlorodiphenyl-trichloroethane (DDT), imidacloprid, and azoxystrobin for pesticides and chemical species of nitrogen, phosphate, and silicate for nutrients. Hydrological flow and transport models of the aquifer were built to simulate groundwater flow and to provide estimates of GLY and dissolved inorganic nitrogen (DIN) fluxes. Stream baseflow was responsible for 59% and SGD for 41% of groundwater flow to the bay, which totaled 6,550 ± 980 m 3 /d in the dry season when surface runoff was negligible. DDT was found in 85% and GLY in 100% of tested samples. SGD and baseflow thus delivered 9 ± 2 g/d of DDT, 0.9 ± 0.2 g/d of GLY, 570 ± 100 g/d of DIN and 840 ± 110 g/d of dissolved inorganic phosphorus (DIP) into Faga'alu Bay. While all pesticide levels are below environmental limits, their presence in baseflow and SGD, which discharge continuously year-round, result in sustained fluxes of GLY and DDT to the reef. The presence of DDT in groundwater decades after its last application confirms its long-term environmental persistence.

The Beach Aquifer Microbiome: Research Gaps and Data Needs

“…Work to date suggests that the above mentioned biogeochemical processes may modulate the flux of chemicals to the coastal ocean (Boehm et al, 2006;Santos et al, 2009;Kim et al, 2012;Santos et al, 2014;Yang et al, 2015;Robinson et al, 2018;Welch et al, 2019;McKenzie et al, 2020) when coupled to the inflow and outflow of groundwater to and from the beach aquifer as well as oceanic forcing (Kim and Heiss, 2021). Indeed some field studies demonstrate elemental fluxes from submarine groundwater discharge of beach aquifers that are on par with those from large nearby river systems (Moore, 2010).…”

Ecomimicry in Indigenous resource management: optimizing ecosystem services to achieve resource abundance, with examples from Hawaiʻi

“…Aquaculture ponds not only drove increases in ecosystem services, they also mitigated ecosystem disservices such as those associated with intensified agriculture. Intensified agriculture such as wetland agroecology can enrich water with nutrients beyond baseline levels (Bremer et al 2018), and it also affects the chemistry of SGD (Welch et al 2019). Without sinks to mitigate eutrophication, agricultural run-off can result in ecosystem disservices by fueling harmful blooms of undesirable phytoplankton and macroalgae in nearby coastal waters; in the worst case scenario, such blooms create hypoxic conditions that lead to fish deaths (Smith et al 2002, Dailer et al 2012, McCoy et al 2017.…”

Review Article: Hawai‘i's Cesspool Problem: Review and Recommendations for Water Resources and Human Health

“…As noted previously, surface water transport of contaminants can be a significant contributor to ocean contamination. For example, Welch et al (2019) utilized field measurements and modeling for a watershed in American Samoa to assess the relative contributions of surface and subsurface sources of ocean contamination. An estimated 59 % of pollution came from surface sources while 41 % were subsurface contributions (Welch et al 2019).…”

“…For example, Welch et al (2019) utilized field measurements and modeling for a watershed in American Samoa to assess the relative contributions of surface and subsurface sources of ocean contamination. An estimated 59 % of pollution came from surface sources while 41 % were subsurface contributions (Welch et al 2019). The authors believe that an integrated surface-subsurface modeling approach might be necessary in Hawai'i.…”