Nitrogen (N) and phosphorus (P) enrichments can stimulate algal growth in drinking water sources, which can cause increased production of disinfection byproduct (DBP) precursors. However, the effect of systematic N and P enrichments on DBP formation and control has not been adequately studied. In this work, we enriched samples from a drinking water source - sampled on April 5, May 30, and August 19, 2013 - with N and P to stimulate algal growth at N : P ratios covering almost five orders of magnitude (0.2-4429). To simulate DBP-precursor removal processes at drinking water treatment plants (DWTPs), the samples were treated with ClO2 followed by alum coagulation prior to free chlorine addition to assess the DBP formation potential (FP). Trichloromethane (TCM) was the predominant DBP formed and the TCMFP was the highest at intermediate N : P molar ratios (∼10 to 50), which corresponded with the peak in algal biomass, as measured by chlorophyll-a (Chl-a). Algal biomass was P-limited throughout the study period, and co-limited by N for the August 19 sampling set. The differences in TCMFP between the raw and treated waters decreased with increasing P amendment, indicating that ClO2 and alum coagulation became less effective for TCM precursor removal as algal biomass increased. This study highlights the impact of nutrient enrichments on TCM formation and control and has implications for nutrient management strategies related to source water protection and for DWTPs that use source waters increasingly enriched with N and P.
Changing rainfall patterns resulting from climate change are predicted to influence cyanobacterial blooms and associated production of toxins and other metabolites, but few studies have demonstrated this link. We explored seasonal and interannual trends in 2-methylisoborneol (MIB) concentrations and the correlation of these data with rainfall and hydrologic data from 2000 to 2013. We also conducted a detailed study of cyanobacterial abundances and MIB during 2 hydrologically extreme years, a prolonged drought in 2007 followed by an extremely wet year in 2008. The goal was to assess whether cyanobacterial abundances and explanatory variables that are often correlated with cyanobacterial abundances were useful predictors of MIB concentration. From 2000 to 2013, mean annual rainfall was negatively related to mean annual MIB (r = −0.50, p = 0.08) and explained 25% of the variation in MIB. In 2007, MIB and cyanobacterial abundance were high, whereas in 2008, MIB and cyanobacterial abundance were low. The total N ∶ total P (TN ∶ TP) mass ratio was strongly related to MIB concentration during the drought and explained 44% of the variation in MIB concentration (r = −0.66, p = 0.003). Our results suggest that rainfall and the TN ∶ TP mass ratio may be useful predictors for MIB in drinking-water reservoirs.
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