The Mezquital Valley system is the world's oldest and largest example with regard to use of untreated wastewater for agricultural irrigation. Because of the artificial high recharge associated with the Mezquital Valley aquifers, groundwater is extracted for human consumption, and there are plans to use this groundwater as a water resource for Mexico City. Thus, this study analyzed 218 organic micro-contaminants in wastewater, springs, and groundwater from Mezquital Valley. Five volatile organic compounds (VOCs) and nine semi-volatile organic compounds (SVOCs) were detected in the wastewater used for irrigation. Only two SVOCs [bis-2-(ethylhexyl) phthalate and dibutyl phthalate] were detected in all the wastewater canals and groundwater sources, whereas no VOCs were detected in groundwater and springs. Of the 118 pharmaceutically active compounds (PhACs) and 7 reproductive hormones measured, 65 PhACs and 3 hormones were detected in the wastewater. Of these, metformin, caffeine, and acetaminophen account for almost sixty percent of the total PhACs in wastewater. Nevertheless, 23 PhACs were detected in groundwater sources, where the majority of these compounds have low detection frequencies. The PhACs sulfamethoxazole, N,N-diethyl-meta-toluamide, carbamazepine, and benzoylecgonine (primary cocaine metabolite) were frequently detected in groundwater, suggesting that although the soils act as a filter adsorbing and degrading the majority of the organic pollutant content in wastewater, these PhACs still reach the aquifer. Therefore, the presence of these PhACs, together with the high levels of the endocrine disruptor bis-2-(ethylhexyl) phthalate, indicate that water sources derived from the recharge of the studied aquifers may pose a risk to consumer health.

The use of compound-specific isotope analysis (CSIA) as a diagnostic tool for MTBE biodegradation in aquifers was tested at the Port Hueneme, CA site. There, a 1500-m long dissolved MTBE plume and associated engineered aerobic flow-through biobarrier have been well-studied, leading to delineation of regions of known significant and limited bioattenuation. This allowed comparison of field-scale CSIA results with a priori knowledge of aerobic MTBE biodegradation, leading to conclusions concerning the utility of CSIA as a diagnostic tool for other aerobic biodegradation sites. Groundwater samples were collected and analyzed for both 13C and 2H (D) in MTBE through the bioactive treatment zone and within the larger MTBE plume. For reference, the 13C enrichment factor for MTBE biodegradation in laboratory-scale microcosms using site groundwater and sediments was also quantified. Aerobic microcosms showed a 13C enrichment of 5.5 to 6.4 +/- 0.2 per thousand over a two-order of magnitude concentration decrease, with an average isotope enrichment factor (epsilon(c)) of -1.4 per thousand, in agreement with other aerobic microcosm studies. Less 13C enrichment (about 25%) was observed for similar MTBE concentration reductions in groundwater samples collected within the aerobic biotreatment zone, and this enrichment was comparable to the scatter in delta13C values within the source zone. Increasing enrichment with decreasing MTBE concentration seen in microcosm data was not evident in either the 13C or D field data. The discrepancy between field and laboratory data may reflect small-scale (<1 m) spatial heterogeneity in MTBE biodegradation activity and the mixing of water from adjacent strata during groundwater sampling; for example, relatively nonattenuated MTBE-impacted water from one stratum could be mixed with highly attenuated/low-MTBE concentration from another, and this could produce a sample with both reduced MTBE concentration and low enrichment. Overall, the results suggest that 13C data alone may produce inconclusive results at sites where MTBE undergoes aerobic biodegradation, and that even with two-dimensional CSIA (13C and D), an increase in the confidence of data interpretation may only be possible with data sets larger than those typically collected in practice.

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