the presence of human and other fecal sources are increasingly powerful tools for
source tracking, but considerable guidance is needed for when and how to best
use them.
The SIPP manual identifies and describes six tiered steps to implement a
program that will identify the sources of fecal pollution, while conserving resources
through progressive deployment [6]. It is not until step 4 that the DNA-based assays
are brought into play, and steps 1–3 should be performed or accounted for prior to
using the assays.
Step 1: Fully characterize the watershed by obtaining infrastructure and topographical maps, interviewing relevant local experts, and conducting visual inspections to develop a list of potential fecal pollution sources in the watershed. This step
should include developing hypotheses about the most likely fecal sources and
conditions under which they contribute pollution to the watershed.
Step 2: Examine all available historical and current FIB monitoring data for
spatial, temporal, and seasonal trends to help identify conditions that result in
elevated FIB levels (e.g., tides, precipitation) and look for any potential linkages
in elevated FIB levels to what appears to be the largest likely sources of fecal
contamination in the watershed. Examples of likely sources include: the presence of
seasonal migratory birds, hot summer days when livestock and wildlife are attracted
to water; large number of seasonal bathers at recreational waters; and combined
sewer overflow (CSO) events associated with precipitation (if a CSO system is
present). At this point, localized intensive sampling for FIB may be appropriate.
Such additional sampling can provide considerable clarification about potential
sources of FIBs and should support or disprove hypotheses developed in step
1. Some reevaluation will be needed for any initial hypotheses that are not
supported. Even though numerous case studies have demonstrated the utility of
progressing through the tiered approach in the step-wise sequence, if intensive
sampling is necessary at this point, then it may be appropriate to jump to step 4 and
employ one or more DNA markers to confirm or disprove the presence of pollution
from a highly suspected source.
Step 3: When leakage or cross-connections from a sanitary system is a potential
source, it should best be investigated at this point using traditional tools such as
smoke testing, tracer dyes, or camera inspection. Smoke and dye testing are also
useful in rural watersheds where leaking or failing septic tank and drainfield
systems may be a problem. This testing may also involve additional sampling for
FIB and might include the human markers, sewage-specific chemicals, or even the
new approach of canine scent tracking. Sewage-specific chemicals include optical
brighteners in laundry detergents, caffeine, fecal sterols that are metabolic
by-products of human digestion processes, and a metabolite of nicotine (cotinine)
excreted by tobacco users. However, these methods are rarely used because they
tend to be quickly diluted to levels below detection limits once the waste stream
enters the ambient environment. Also, the cost to measure the organic chemicals is
high and requires both specialized equipment and trained personnel. However, for
those with the necessary instruments for detailed chemical analysis, chemical
methods might be useful for independently confirming human fecal sources in a
Microbial Source Tracking: Advances in Research and a Guide to Application
279
source tracking, but considerable guidance is needed for when and how to best
use them.
The SIPP manual identifies and describes six tiered steps to implement a
program that will identify the sources of fecal pollution, while conserving resources
through progressive deployment [6]. It is not until step 4 that the DNA-based assays
are brought into play, and steps 1–3 should be performed or accounted for prior to
using the assays.
Step 1: Fully characterize the watershed by obtaining infrastructure and topographical maps, interviewing relevant local experts, and conducting visual inspections to develop a list of potential fecal pollution sources in the watershed. This step
should include developing hypotheses about the most likely fecal sources and
conditions under which they contribute pollution to the watershed.
Step 2: Examine all available historical and current FIB monitoring data for
spatial, temporal, and seasonal trends to help identify conditions that result in
elevated FIB levels (e.g., tides, precipitation) and look for any potential linkages
in elevated FIB levels to what appears to be the largest likely sources of fecal
contamination in the watershed. Examples of likely sources include: the presence of
seasonal migratory birds, hot summer days when livestock and wildlife are attracted
to water; large number of seasonal bathers at recreational waters; and combined
sewer overflow (CSO) events associated with precipitation (if a CSO system is
present). At this point, localized intensive sampling for FIB may be appropriate.
Such additional sampling can provide considerable clarification about potential
sources of FIBs and should support or disprove hypotheses developed in step
1. Some reevaluation will be needed for any initial hypotheses that are not
supported. Even though numerous case studies have demonstrated the utility of
progressing through the tiered approach in the step-wise sequence, if intensive
sampling is necessary at this point, then it may be appropriate to jump to step 4 and
employ one or more DNA markers to confirm or disprove the presence of pollution
from a highly suspected source.
Step 3: When leakage or cross-connections from a sanitary system is a potential
source, it should best be investigated at this point using traditional tools such as
smoke testing, tracer dyes, or camera inspection. Smoke and dye testing are also
useful in rural watersheds where leaking or failing septic tank and drainfield
systems may be a problem. This testing may also involve additional sampling for
FIB and might include the human markers, sewage-specific chemicals, or even the
new approach of canine scent tracking. Sewage-specific chemicals include optical
brighteners in laundry detergents, caffeine, fecal sterols that are metabolic
by-products of human digestion processes, and a metabolite of nicotine (cotinine)
excreted by tobacco users. However, these methods are rarely used because they
tend to be quickly diluted to levels below detection limits once the waste stream
enters the ambient environment. Also, the cost to measure the organic chemicals is
high and requires both specialized equipment and trained personnel. However, for
those with the necessary instruments for detailed chemical analysis, chemical
methods might be useful for independently confirming human fecal sources in a
Microbial Source Tracking: Advances in Research and a Guide to Application
279