OPTIMIZATION OF A SPECTROPHOTOMETRIC METHOD TO DETERMINE AMMONIA IN GROUNDWATER TO UNDERSTAND ARSENIC OCCURRENCE IN SHALLOW AQUIFERS OF CHITTAGONG, A RAPIDLY EXPANDING COASTAL CITY OF BANGLADESH

High concentrations of arsenic in groundwater have been reported from different regions in the world. Serious health effects due to elevated arsenic consumption have been documented in several South Asian countries. However, the link between arsenic and other ions in shallow aquifer waters containing much higher nutrients (TOC, TN, NH₄ and P) remain poorly understood. In the study area of Chittagong, Bangladesh, the northern part of the city is a part of Anticline associated with Tertiary aquifers and the southern part of the city is plain land associated with Quaternary aquifers. Preliminary studies have reported elevated arsenic and manganese (Mn) in those aquifers. However, as to date, no study has been conducted to investigate the relation between arsenic and nutrients to understand the geochemical processes that control the As distribution in groundwater. This study reports the modification of a spectrophotometric inexpensive reliable method to measure the ammonia concentration in a set of several groundwater samples collected from both shallow and deep aquifers in Chittagong area. The preliminary results showed high level of ammonia is associated with elevated level of As and Mn, suggests that reducing condition may control the As and Mn release in groundwater. The results suggest that the role of nutrients on the microbial activity and the mobility of As in both aquifers warrant further investigation.