Submarine groundwater discharge and nutrient addition to the coastal zone of the Godavari estuary

Abstract

Submarine Groundwater Discharge (SGD) represents a significant pathway of materials between land and sea, especially as it supplies nutrients, carbon and trace metals to coastal waters. To estimate SGD fluxes to the Godavari estuary, India, we used multiple tracers: salinity, Si, ²²³Ra, ²²⁴Ra, ²²⁸Ra and ²²⁶Ra. Tracer abundances were elevated in groundwater from the unconfined coastal aquifer and in surface water from the near shore zone; these enrichments decreased to low levels offshore, indicative of groundwater discharge. A model based on the decay of ²²⁴Ra relative to ²²⁸Ra was used to determine apparent water ages of various bays within the estuary. These ages ranged from 2.6 to 4.8 d during November 2011. Knowing the water age, the distribution of radium in the estuary and the radium isotopic composition of groundwater enabled us to calculate SGD fluxes to the estuary. These fluxes (in units of 10⁶ m³ d⁻¹) were on the order of 5 in the Gautami Godavari estuary, 20–43 in the Vasishta Godavari estuary and about 300 in Kakinada bay, where enhanced ion exchange processes and redox-controlled cycling in the mangrove ecosystem may contribute to higher fluxes. These estimates of water fluxes allowed us to determine the magnitude and seasonal variability in the nutrient fluxes to the estuary associated with SGD. These nutrient fluxes (in units of mmol m⁻² d⁻¹) ranged from 1–19 (N), 0.6–2.6 (P) and 5–40 (Si) in Gautami Godavari; 19–40 (N), 2.6–5.5 (P) and 200 (Si) in Vasishta Godavari; and 120–140 (N), 10 (P) and 220 (Si) in Kakinada bay. The high SGD fluxes to Kakinada bay contribute significant nutrients to this bay; considerably lower SGD fluxes to Vasishta Godavari still contribute significant nutrients to this estuary. Thus SGD represents a major source of new nutrients to these coastal ecosystems. For the entire Godavari estuarine system, SGD fluxes contribute (48–88) × 10⁹ mol DIC y⁻¹ and (51–94) × 10⁹ mol TA y⁻¹. These fluxes represent ∼ 54 and ∼ 62% of the riverine DIC and TA fluxes to the Godavari estuarine system. This study provides baseline data against which future changes in nutrient and carbon fluxes due to urbanization and economic growth over this region can be compared.