Contaminant source tracking can be used for identifying the specific source of certain microbial and chemical contaminants, and for eliminating potential sources from consideration. Chemical or microbial 'fingerprints' can be analyzed and compared for identifying sources of those contaminants.
References that summarize the capabilities and state-of-the-science of microbial source tracking include the following:
Chemical Source Tracking:
For chemical contaminants, various environmental sources have distinctive elemental isotope ratios for elements, and environmental processes can have impact on isotopic levels and ratios (Kendall, 2004; Lindsey and Koch, 2004; Moore et al.,2006). One example use of isotopic analysis in agricultural settings is tracing the source of nitrate found in groundwater through analysis of isotopes of nitrogen and perhaps oxygen. For example, soil nitrate may be compared to nitrate from an animal waste source through analyses of N15. There is basically none of the N15 isotope in commercial fertilizer, but it is present in animal and human waste. However, this method does not usually work to distinguish septic waste from animal manure or to differentiate among manure from different kinds of farm animals, and in these cases a multi-isotope or multi-tracer approach would be better. N14 denitrifies faster than N15, so in groundwater systems one would need to distinguish the denitrification process from the source. Some options that are used less frequently are isotopic analyses of nitrogen (N15) in dissolved organic nitrogen and ammonia, and oxygen (O18) in phosphate or dissolved oxygen. USEPA, USDA, USGS, and many universities have isotope laboratories that can provide isotopic analytical services and possibly collaborations (Kendall, 2004).
Microbial Source Tracking:
(Note: Thierry Sam Tamers and Troy Scott of Source Molecular Corporation (Miami, Florida) wrote much of this section on microbial source tracking)
"Microbial source tracking" (MST) is a rapidly expanding science designed to identify human or animal sources of fecal pollution in the environment. These methods can evaluate E. coli, Giardia, Cryptosporidium, and viruses, and identify whether the source is from human, cattle, swine, bird, or other origin. Fecal source tracking using DNA "fingerprinting" can be used by water utilities to identify whether or not particular a potential fecal source in their watershed, such as an AFO or CAFO, is an actual contributor to wastes identified in the source water. Microbial source tracking analyses can cost a few hundred dollars each, so the sampling program design should be carefully planned and targeted.
Currently, the standard for most microbiological water quality monitoring utilizes the identification and enumeration of traditional and alternative microbial fecal indicator organisms. These indicators include total coliforms, fecal coliforms, E. coli, enterococci, and several other microorganisms that are associated with the human and animal intestinal tract. The premise of this approach is that these intestinal bacteria may also indicate the presence of other human and animal fecal pathogens that reside in the intestinal tract and may pose a risk to human health. The caveat to the indicator approach is that recent research has shown these indicators have limited utility for this purpose, citing differences in ecology, prevalence, resistance to stress, and the existence of environmental reservoirs in areas with no known fecal input. The result is a water quality monitoring system that may fail to accurately predict potential health impacts and offers no tools or information regarding remedial actions (Scott et al., 2002).
Source tracking methodologies take the indicator approach one step further by identifying the source or sources of these organisms. By identifying specific sources, better conclusions can be drawn regarding the possible presence of other pathogenic bacteria, viruses, and parasites. In addition, once a source is implicated, more appropriate steps can be taken to remedy the problem. The success (or failure) of a source-tracking project lies in the types of methods employed, the specific approaches taken, and the types of analytical tools used to interpret the results.
To further help guide water utilities to best use microbial source tracking to their advantage, the process by which a source tracking study should be developed is outlined below:
- Specific knowledge of the watershed and potential fecal inputs is a critical component of a successful source tracking study. By concentrating only on potential inputs, the methods employed can be specifically targeted to identify these sources.
- All methods should be employed in a tiered fashion and results should be interpreted sequentially so as to implicate or eliminate potential sources one at a time.
- The sampling approach should be well planned and should be conducted during both high flow (wet) and low flow (dry) events. Care should also be taken to perform targeted sampling, where samples are taken both upstream and downstream of potential fecal inputs. If bacterial counts are low upstream and high downstream (and source tracking data indicate a strong correlation with the suspected source(s)), then this is a strong indication that the source has been identified. Conversely, this approach can also eliminate suspected sources and redirect attention to other potential impact sites.
- End-users are encouraged to adopt a "toolbox" approach to any source tracking study. By using multiple methods that target and identify different source indicators, more comprehensive conclusions can be drawn. The most important aspect of the toolbox approach is that individual results can be confirmed and substantiated. Conclusions regarding potential inputs should never be drawn due to the results of a single test on a single water sample. On the contrary, multiple positive results using a variety of methods are a strong indicator that the source(s) of the pollution have been identified.
- If budgetary constraints are present, end users are encouraged to conduct their source tracking studies in a piecemeal fashion. It is better to target one or two potential sources with several tests, rather than to try to pinpoint all the potential sources with only one test per suspected source. Most source tracking projects fail to render meaningful results because too many sources of fecal pollution are targeted simultaneously. By limiting the number of potential targets, the end user can work by a process of elimination towards the likeliest sources of contamination.