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