Balancing Health Risks (Nitrate vs THMs) from Surface and Ground Water Sources

摘要

Field tracer experiments and modeling work were conducted in order to evaluaternthe possibility to augment surface water supplies with ground water supplies.rnThis situation is necessitated in the southwest by the potential for short-termrndroughts (3-5 years) that can significantly affect surface water supplies. Bothrnnitrate and trihalomethane (THM) concentrations in finished drinking water canrnbe managed through optimizing well pumpage. While there are excess cancerrnhealth risks associated with THMs, the maximum contaminant level for nitrate isrnbased upon a level suitable to prevent methomeogloulinemia in children.rnTherefore municipalities must set an acceptable risk of moderate nitrate levels (3-rn6 mg/L) if they want to tap into water supplies that contain nitrate. Fieldrnmonitoring and estimation of nitrate uptake rates by algae and biota within thernopen canal delivery system suggest that nitrate uptake will generally be less thanrn1 mg/L, and therefore can not be considered as a reliable in-stream treatmentrnalternative. Steady-state modeling was undertaken, and the results demonstraternthat while operating wells may slightly increase the nitrate levels (still < 5 mg-rnN/L) entering a water treatment plant, the low DOC groundwater should producernfewer THMs after treatment. However, the well water does contain elevatedrnbromide levels (>1 mg/L) which tends to shift the speciation of THMs towardrnbrominated compounds. The results from a steady–state modeling approachrnprovide a basis for developing a dynamic hydraulic simulation model that canrntrack nitrate, DOC, and bromide through a complex water supply delivery system.