A Study of the Precursors for Disinfection By-Products on the CAP Avra Valley Recharge Project

Abstract

Disinfection by-products (DBPs) can form when natural organic matter (NOM) reacts with chlorine disinfectants and bromide ions found in natural waters. This study was conducted at the Central Arizona Water Conservation District's Avra Valley Recharge Project site. Water samples were collected from recharge basins, wells and piezometers at the site during a fifteen-month period. The NOM was measured as TOC and DOC in untreated samples, whereas DBP-formation-potential was measured as the sum of five trihalomethanes and four haloacetic acids in disinfected water. Seven other inorganic constituents were also measured to quantify water quality changes associated with recharge. Significantly lower NOM concentrations were observed for the samples collected from recovery wells that had TDS and chloride levels similar to the Central Arizona Project (CAP) water source. However, the NOM concentrations in vadose zone water samples were much closer to the levels in CAP water. This suggests that some NOM is being removed from the CAP water during recharge though processes other than dilution (mixing). Recovered and disinfected recharged CAP water produced proportionally (relative to NOM) lower but significant amounts of DBPs in the vadose zone and groundwater water samples. The data from this study and from two other CAP recharge projects suggest that NOM levels will continue to increase in the vadose and aquifer zones influenced by recharged CAP water.