ANALYSIS OF MIXING BETWEEN RIVER WATER AND GROUNDWATER ALONG THE BANKS OF A LARGE TIDAL RIVER RECEIVING AS-LADEN GROUNDWATER

Groundwater is the main source of drinking water in Bangladesh where the shallow alluvial aquifers (<50 m) typically have toxic concentrations of geogenic arsenic (As). The Meghna River, one of the main rivers in the country, gains water from these aquifers for most of the year, but As concentrations in the river are near detection limit. In addition to dilution, As may be sequestered and naturally accumulate in the river’s banks. In sandy banks a dynamic mixing zone between Fe-reducing groundwater and oxygen-rich surface water operates during the dry season, driven by tidal and seasonal fluctuations. These reactive mixing zones are potentially active sinks and sources of As. The objective of this study is to estimate the conservative mixing between groundwater and river water which will help predict the fate and transport of As.

Fifty-two groundwater and two river water samples were collected from the field site. Labile parameters were measured on site. Dissolved As and iron (Fe) were measured with Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Major cations and anions were analyzed using Ion Chromatograph (IC) and the water isotopes (δ¹⁸O and δD) were analyzed using Cavity Ring-down Spectroscopy (Picarro L2120-i).

The arriving groundwater was only slightly affected by evaporation prior to recharging the aquifer, but not from evaporation of the upwelling groundwater along the seepage face of the riverbank.

Concentrations of dissolved As, Fe, phosphate, alkalinity, and ammonium increased towards the river, while δ¹⁸O, chloride (Cl), and specific conductance decreased towards the river. This indicates an active source of these well-known by-products of organic matter decomposition and reductive dissolution of Fe oxides.

We applied an end-member mixing model based on Cl concentrations and δ¹⁸O to quantify mixing between groundwater and river water. This allowed the mass fluxes of As, Fe, phosphate, alkalinity, and ammonium to be calculated along the flow path towards the river.

Future work using geochemical inverse modeling will help identify the chemical reactions responsible for the observed fluxes and concentrations. Understanding coupled hydro-biogeochemical processes in riverbanks in regions with high concentrations of As in shallow aquifer will shed new light on As and Fe cycling across the basin.