Groundwater Quality Restoration and Coastal Ecosystem Productivity

Abstract

Groundwater is defined as water stored underground, in rock and soil pore and fracture space. About 99% of all fresh liquid water is groundwater. It covers ~30% of human freshwater needs, with ~70% used in agriculture. However, groundwater quality is declining worldwide. Contaminated groundwater is shown to be an important stressor in marine habitats, with multiple negative consequences to ecology, ecosystem function and the provision of societal goods and benefits. To assess the need to restore groundwater quality to preserve the productivity of coastal ecosystems, we review the contribution of submarine groundwater discharge (SGD) to coastal nutrient budgets and discuss the impact of SGD on eutrophication and eutrophication mitigation plans. We find that roughly 14 % of the total nitrogen (N) and 3.9 % of the phosphorus (P) annual inputs into agroecosystems arrives at sea via SGD. This transfer is modulated by subterranean estuaries (STEs), underground zones named after their surface analogs where saline and fresh groundwater mix. Total (fresh + saline) SGD contributes an average of 2.3, 0.06 and 3.8 Tmol yr−1 of total dissolved N, P and Silicate (Si), respectively to the coastal ocean (N:P:Si ratio of 38:1:63). This flux is comparable to that of global riverine input of 2.32, 0.08 and 6.42 Tmol yr−1 of total dissolved N, P and Si into the ocean (N:P:Si ratio of 29:1:80). The flux of groundwater nitrogen closes the gap between sources and sinks in the coastal N cycle to between −0.96 and −3.91 Tmol N yr−1, helping SGD support ~2% (15.2 Tmol C yr−1) of coastal Net Primary Production (NPP). These fluxes make groundwater borne nutrients an important load pressure on coastal ecosystems. Subterranean estuaries function as particulate filters and further uncouple N from P and Si loading, making groundwater contribute to significant enrichment of coastal ecosystems in N by comparison to P and Si. The impact of nutrient enrichment of coastal systems by SGD will be amplified in both time and space by the relatively slow movement of water through soils and permeable rock, compared to open water discharge. Indeed, the relatively slow pace of transfer of mass through the sub-surface pathway results in a transient retention of nutrients in transit to sea in continental aquifers and soils. As a result, the effect of groundwater discharge on marine habitats lags behind the contamination of groundwater, sometimes by decades. This ‘time lag’ depends on the hydrogeology and biogeochemistry of aquifer systems and is thus regionally distinct. Time lags are poorly constrained worldwide, and yet cause the visible effects of groundwater nutrient pollution mitigation programs to lag implementation by years to decades. Furthermore, because SGD is ignored in current assessments of eutrophication status in coastal ecosystems, feedbacks between legacy pressures (i.e. / e.g., the nutrients accumulated in coastal groundwater bodies as a result of decades of intensive agriculture and already in transit to sea), SGD, surface runoff and coastal eutrophication are not understood. Long term strategies for nutrient (N, P) reduction, coupled to monitoring of water quality of both surface and subterranean estuaries and their microbial populations, are necessary to preserve coastal productivity and ecosystem health for future generations. The prioritization of these strategies should be informed by groundwater age profiling of coastal aquifers. This information will lead to a better management of lag times between the implementation of catchment nutrient management strategies and their outcomes in terms of coastal eutrophication reduction.