Abiotic hydrolysis of simazine, atrazine, diazinon, methylparathion, and
chlorpyrifos was studied
in three different natural waters and buffered Milli-Q water. The
triazines showed no detectable
decrease in concentration in any of the waters over 43 days at pH 8.0
and 40 °C. The rates of
hydrolysis for diazinon and methylparathion were statistically similar
in all waters tested.
Chlorpyrifos exhibited a ∼32% decrease in hydrolysis rate in
the presence of a high concentration
of dissolved organic matter (DOM) (34.5 mg/L dissolved organic carbon).
The activation energies
are larger, and thus the predicted hydrolysis rates are significantly
slower than those previously
reported. The effect of continuous vigorous mixing on hydrolysis
rates was investigated and found
to have only a minor effect. The results suggest that uncatalyzed
abiotic hydrolysis is very slow for
these compounds at the temperatures and pH's typical of most natural
waters and affirm the need
for a greater understanding of the relative influence of DOM,
catalysis, and biodegradation on the
fate of organophosphate and triazine pesticides.
Keywords: Pesticides; organophosphates; triazines; hydrolysis; abiotic;
DOM
The Santa Ana River (SAR), CA and adjacent wetlands have been identified as potential sources of fecal indicator bacteria (FIB) to the surf zone at Huntington Beach, CA. A suite of fecal steroids, including coprostanol (COP), epicoprostanol (eCOP), cholesterol (CHOE), cholestanol (CHOA), alpha-cholestanone (aONE), beta-cholestanone (bONE), beta-sitosterol (bSIT), stigmasterol (STIG), stigmastanol (STAN), and campesterol (CAM), were used as chemical markers to examine whether sewage was a significant source of FIB within the lower Santa Ana River watershed. A total of 54 water samples were collected from three locations in the intertidal zone near the mouth of the Santa Ana River at different tidal stages. Steroid ratios in SAR samples were different from those found in raw and treated sewage from a local wastewater treatment plant or in nearby effluent plume and did not appear to be influenced by the sampling location, daily tides, and spring/neap tidal cycle. The characteristics of steroid ratios suggested a diagenetic ratherthan a biogenic source forthe COP content of the samples. The log-based concentrations of COP and FIB in the SAR samples were not significantly correlated, inconsistent with sewage being the source of FIB in the study area. In addition, multivariate statistical analysis showed that the concentrations of FIB were better correlated with bird fecal steroids than with the typical sewage sterols. The results implied that sewage was not a significant source of fecal steroids, and therefore perhaps FIB to the study area. Instead, birds may be one possible source of the intermittently high levels of FIB observed in the lower Santa Ana River watershed and the nearby surf zone.
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