Determining an appropriate set of indicators to show that a water supply is not impacted by wastewater is critical for watershed protection and for potable reuse projects. In 2010 the California State Water Resources Control Board (CA SWRCB) Blue Ribbon Panel issued a set of recommendations for a very short list (7) of compounds to be monitored for re-use projects. In 2009/10 the Santa Ana Watershed Project Authority (SAWPA) developed a separate list of potential wastewater indicator compounds (11) based on a consensus panel, with the intention of sampling all dischargers to the Santa Ana River to determine occurrence. In 2009, the Water Research Foundation (WaterRF) funded project 4167, which examined analytical performance for 22 potential compounds of interest for assessing drinking water. In 2010, a NWRI report suggested primidone as a tracer. In 2009 Buerge et al. demonstrated the effectiveness of artificial sweeteners as conservative wastewater tracers. There is some overlap among these lists but there are also some potential key indicators that are only on a single proposed list and the potential that some are on ‘none’ of the lists.

MWH developed a direct online extraction/analysis method using LC-MS-MS for nearly 90 target analytes that covers all of the compounds listed in the projects above. This method has reporting limits for most of the analytes in the 5–10 ng/L range. In 2010 and again in 2011, this technique was used for a SAWPA effluent monitoring project to assess the relative amount of information obtained from the different lists. Samples from 17 wastewater effluents and the downstream receiving water were tested for ∼90 analytes. We compared (a) frequency of detection in the effluent samples (b) concentration range and (c) variability among dischargers. Data were compared to other locations collected across the country and to municipal drinking water sources and distribution system samples to determine the most persistent compounds (and therefore best indicators). The ideal generic tracer should be relatively abundant and show minimal concentration variation.

It is apparent that none of the existing lists (SAWPA, SWRCB, WaterRF, primidone, sucralose/acesulfame-K) is a ‘perfect’ list and greater discerning power is obtained by looking at long lists. Specifically the SAWPA list suffers because only one compound, TCEP was detected in all effluent samples, and concentration varied by a factor of 10 among dischargers. The CA SWRCB list suffers because only sucralose was detected in all samples; iopromide was only detected in about 50% of the samples and gemfibrozil in an even smaller number of effluents. Primidone, while occurring in 100% of samples varies by five fold among local sources and thus may not be the ideal tracer. Many of the compounds in the WaterRF list are not even among the most commonly detected analytes in effluents.

Taking advantage of the power of a long list covering various classes of compounds it becomes possible to identify both generic (sucralose, acesulfame-K, TCEP, primidone) and specific (iohexal, atenolol, butalbital, caffeine degradates such as theobromine and 1, 7-dimethylxanthine, triazine and degradates) tracers to not only determine the presence of wastewater, but also potential specific primary wastewater sources. Several of the compounds seen most frequently that also have less than ten fold variation include ones not commonly measured (Dehydronifedipine, dilantin, meprobamate). Once the appropriate target analytes are identified for a given set of dischargers, future monitoring can then use a smaller set, but a-priori determination of a short list of indicators may eliminate significant important potential tracer compounds.

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