Contamination of dugwell water and its control

Abstract

Water quality studies conducted so far have shown that dug wells have reduced arsenic ingestion but exposed population to high levels of health risk from microbial contamination. This study aims at understanding the nature of contamination of dugwell water and decontamination by in-situ chemical disinfection. For preliminary water quality analysis, dugwells were selected from Sirajdikhan, Singair, Daudkandi and Sharsha upazilas. After preliminary water quality analysis, two dugwells with high microbial contamination from Sirajdikhan and one dugwell with high arsenic content from Sharsha were selected for decontamination study. Natural growth/decay of total coliform and faecal coliform was studied in the laboratory and break point chlorine dose was determined for the samples under consideration. Then, starting from the break point dose, different doses were applied to the dugwell water in the laboratory and the chlorine dose for complete removal of coli forms from the water was determined. The effect of chlorination on other water quality parameters like arsenic, ammonia, manganese, iron, turbidity, etc. was also studied in the laboratory. Chlorination was performed in each of the selected dugwells and continued for a couple of days. Water samples were collected from each of the dugwells both before and after chlorination and analyzed for selected water quality parameters on each day of chlorination. Finally, recharge capacity of 51 dugwells in Sharsha upazila was studied and user acceptability surveyed at Sirajdikhan. Rate of microbial decay in the dugwell water has been found to be significantly high (maximum of0.455/day). The negative or very low Eh (-102 mY) and low dissolved oxygen (0 mg/I) of dug well water are not favourable for oxidation of iron, arsenic and odour producing substances. Dissolved arsenic (as high as 0.14 mg/I at Sharsha), iron (as high as 10 mg/I at Daudkandi) and manganese (as high as 1.48 mg/l at Sirajdikhan) were present in dugwells. There was hardly any difference between the water quality of open and closed dug wells and the efforts made for keeping the dug wells open seemed futile. The improved dugwells produce an average of 2.09 m3/day of water in the wet season and 0.58 m3/day of water in the dry season, which are inadequate to meet the requirements. Residual chlorine in the range between 0.5 to 1 mg/I destroyed all coli forms. However, the chlorine level quickly decreased with the inflow of new groundwater in the dug well and rendered the well vulnerable to renewed contamination. Chlorine dosing increased redox potential of dug well water from soluble reducing fields to highly oxidizing fields in Eh-ph phase-stability diagrams of both Fe and As. Although, the oxidizing condition prevails in the DW, following chlorination, the As and Fe concentration increased in the collected samples which is contradictory to the theory. However, this might have happened as the Fe flocs formed due to oxidized condition settling downward being pumped directly through the uptake pipe and thus, increasing the Fe and As level in the samples. This dynamic flow condition might have caused the effects opposite to those found in theory as well as in the laboratory. In spite of some aesthetic water quality problems, dug well water appeared to be acceptable to most of the respondents at Sirajdikhan. Most of the people in the study areas had no complain about drinking chlorinated water at the levels of 0.5 - 1.0 mg/l of residual chlorine in water. But, intermittent dosing of chlorine even at an interval of once a day has been found to be ineffective in maintaining the coliform to desired level of zero.