XRF AND GEOCHEMICAL STUDY OF PORE FLUID AND SHALLOW SEDIMENTS FROM ARSENIC-SAFE AND ARSENIC-BEARING AQUIFERS IN BANGLADESH

Arsenic contamination in groundwater is a common phenomenon, and thus a grave concern in many of deltaic plains around the world. The Bengal foreland basin drained by the Ganges, Brahmaputra, and Meghna mega river systems is also no exception. The large population density in Bangladesh, its lack of proper water management and execution just aggravated the situation over time. In this research, we analyzed pore fluid samples from the contact of lower end shallow aquitard and the uppermost shallow aquifer collected from arsenic-safe and arsenic-contaminated locations. We also conducted X-Ray Fluorescence (XRF) analysis on shallow core aquifer sediments to document sediment composition.

The Piper diagram indicates that the collected 41 shallow pore fluid samples from six districts along an east-west transect of central Bangladesh is mostly Ca-HCO₃ type. The active biodegradation of organic matter at shallow depths could be the potential reason for the occurrence of high dissolved HCO₃ in pore fluid. However, we have encountered one sample from Narayanganj district, which belongs to Na-Cl-SO₄-HCO₃ type while two other pore fluid samples from Shirajganj and Kustia districts represent Ca-SO₄ type. The presence of Na-Cl-SO₄-HCO₃ type signifies more of a marine signature, and the original pore fluid was likely mixed with fluids residing in clay rich formation. The occurrence of highly dissolved SO₄ indicates either the oxidation of pyrite, dissolution of SO₄bearing minerals, or the inputs from fertilizer use.

The XRF analysis from the collected shallow aquifer sediments (21'1''-22'1'' below ground level-bgl) shows the high arsenic concentration in Narayanganj district, which is consistent with the elevated arsenic concentration (197.5 µg/L) measured from pore fluid sample of the same site (25' bgl). However, the results from other sites across Bangladesh are not consistent with this pattern, not even at a nearby location in Narayanganj. Arsenic concentration found to be higher at shallow sediments (2'3.5''-6'10'' depth range) than that of the deep sediments (16'-19' depth range) collected from the same borehole in Narayanganj, Chandpur, and Shirajganj districts. Ongoing studies involving X-Ray Diffraction and electron probe will provide additional information on composition of Holocene strata of Bangladesh.