Groundwater-derived nutrient inputs to the Upper Gulf of Thailand

Abstract

We report here the first direct measurements of nutrient fluxes via groundwater discharge into the Upper Gulf of Thailand. Nutrient and standard oceanographic surveys were conducted during the wet and dry seasons along the Chao Phraya River, Estuary and out into the Upper Gulf of Thailand. Additional measurements in selected near-shore regions of the Gulf included manual and automatic seepage meter deployments, as well as nutrient evaluations of seepage and coastal waters. The river transects characterized the distribution of biogeochemical parameters in this highly contaminated urban environment. Seepage flux measurements together with nutrient analyses of seepage fluids were used to estimate nutrient fluxes via groundwater pathways for comparison to riverine fluxes.

Our findings show that disseminated seepage of nutrient-rich mostly saline groundwater into the Upper Gulf of Thailand is significant. Estimated fluxes of dissolved inorganic nitrogen (DIN) supplied via groundwater discharge were 40–50% of that delivered by the Chao Phraya River, inorganic phosphate was 60–70%, and silica was 15–40%. Dissolved organic nitrogen (DON) and phosphorus (DOP) groundwater fluxes were also high at 30–40% and 30–130% of the river inputs, respectively. These observations are especially impressive since the comparison is being made to the river that is the largest source of fresh water into the Gulf of Thailand and flows directly through the megacity of Bangkok with high nutrient loadings from industrial and domestic sources.

Introduction

Groundwater is an important source of nutrients and other dissolved constituents to the coastal ocean. In some areas groundwater inputs may rival those of nearby rivers although few comparisons are available. Estimates of the location and magnitude of groundwater discharge are scarce, because measurements cannot be performed easily and sites of discharge are not always obvious—groundwater is the “invisible pathway” between land and sea. Since the chemical composition of groundwater and its rate of discharge determine the net effects on the overall chemical budget of receiving waters, the contribution will be significant when either flow rates or the ratio of components in groundwater relative to the surface waters is sufficiently high (Moore, 1996, Moore, 1999, Zektser, 2000, Burnett et al., 2003, Buddemeier, 1996).

The direct discharge of groundwater into the coastal zone, called “submarine groundwater discharge” (SGD) is now recognized as a significant, but poorly quantified, pathway between land and sea. As such, SGD acts as a source of inorganic and organic carbon, nutrients, and other dissolved species to coastal waters and ecosystems. The term “groundwater” as used here may be either meteorically derived fresh water, seawater that has infiltrated the subsurface and reacted with aquifer solids (referred to subsequently as “saline groundwater”), or a combination (Burnett et al., 2003). SGD is very difficult to quantify because of its diffusive nature—perhaps the main reason the process had been ignored by coastal scientists for so long. Significant progress has been made in the last decade in advancing our understanding of the forces and components of SGD as well as advances in measurement techniques, protocols that can be applied elsewhere, and models (Moore, 1996, Moore, 1999, Burnett et al., 2002, Taniguchi et al., 2002, Burnett et al., 2003, Slomp and Van Cappellen, 2004). A review of all SGD research performed as of a few years ago showed that there is a significant lack of data for regions in Africa, South America, and Southeast Asia (Taniguchi et al., 2002). One conclusion of that review was that groundwater transport could be particularly important in Southeast Asia because of the wet humid climate and excessive limestone terrains that are conducive to rapid underground flow.

We report here our initial assessments of seepage rates from two areas of the Upper Gulf of Thailand. We will show that while the fresh groundwater discharge component is generally low in this region, saline groundwater fluxes of both inorganic and organic forms of nitrogen and phosphate are substantial. Our estimates indicate that these fluxes are on the same order as those from the Chao Phraya River, the principal source of fresh water discharge to the Gulf of Thailand.