Environ. Sci. Technol. 1996, 30, 1872-1881
Coliform Contamination of a Coastal Embayment: Sources and Transport Pathways PETER K. WEISKEL* Department of Earth Sciences, Boston University, Boston, Massachusetts 02215
BRIAN L. HOWES Biology Department, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543
GEORGE R. HEUFELDER Barnstable County Health & Environmental Department, Barnstable, Massachusetts 02630
Both laboratory and field experiments suggest significant elution of bacteria from wrack, ∼3 × 1012 FC yr-1 on a bay-wide basis (6% of annual input), primarily by periodic tidal flooding and possibly by major rain events. Release of coliforms during resuspension of subtidal sediments was estimated to be a minor source in this system (2.5 cm of rainfall in 96 h) are only about 4% of the associated rainfall volume over the stream watersheds. During June-October 1989, 83% of the total streamflow to the bay occurred under dry-weather conditions; the remaining 17% was associated with seven major rain events (Table 2). Although these wet-weather events were relatively infrequent, streamflow during these events (stormflow + baseflow) had FC densities about 6 times higher than the dry-weather flows (Table 2, Figure 5). The higher wet-weather flows and associated FC densities in the 1989 study period resulted in an almost 9-fold higher daily FC load from streams in wet vs dry weather. More generally, the elevated FC concentrations regularly observed at the stream mouths in the spring and summer (Figures 3 and 5 and unpublished data, Commonwealth of Massachusetts) are likely caused by (1) the increased wetweather flows associated with precipitation events in the warm months, when temperatures are always above freezing, (2) possible warm season increases in wildlife FC loading to watershed surfaces (see below) contributing runoff to the streams, and (3) the possible effect of temperature-induced stress on indicator survival and (or) culturability in the winter samples.
The major sources of bacteria to the streams in the watershed almost certainly vary as a function of weather conditions. Red Brook especially has long reaches of bordering tidal and nontidal wetlands (Figure 1), which generate runoff during storm events. Stormwater entrainment of animal bacteria from saturated wetland surfaces is a known mechanism for bacterial transport to the streams draining them, as has been observed in a wetlanddominated, coastal watershed free of development-related bacterial sources (32). This appears to be the most likely mechanism for enhanced FC loading during wet weather, particularly in watersheds underlain by highly permeable soils that otherwise generate little stormwater runoff (33). Seasonal variations in wildlife FC loads to contributing wetland surfaces, while not measured in the present study, may help explain the strong seasonal variation in streammouth FC densities noted above (Figure 3). In contrast, bacterial sources to the streams under dryweather conditions are more difficult to identify. However, dry-weather sources are important to consider since they contribute about half of the total stream-related FC load (Table 2). Potential mechanisms for FC input during dry periods include groundwater discharge, runoff associated with minor rain events, and periodic release of bacteria from protected reservoirs within the bay and along tidallyinfluenced reaches of the tributary streams where temperature, light, and (or) nutrient conditions favor enhanced bacterial survival. Groundwater Pathway. Groundwater transport represents a large potential bacterial transport pathway to the waters of Buttermilk Bay. About 85% of the annual freshwater influx to the Bay consists of either direct groundwater discharge or groundwater discharge to streams that flow into the bay (20). Moreover, groundwater underlying the residential areas adjacent to the bay is heavily contaminated with nitrate from on-site septic systems (34, 35), which may also contribute bacteria to the groundwater flow system. While groundwater contamination from animal sources is possible, especially from livestock in agricultural areas (4), our background data from sites not directly affected by septic system plumes suggested that animals do not contribute significant numbers of bacteria to the groundwater of the study area. Therefore, we focused on septic systems as potential bacterial sources for the groundwater pathway. Fecal coliform density in effluent discharged from the four septic systems was log-normally distributed, with a geometric mean of 105.07 FC 100 mL-1 (SEM ) ( 0.1 log units; Figure 6). In strong contrast to the effluent, groundwater from wells within the core of each septic system plume 1 m downgradient of each leaching structure showed very low FC densities (Figure 6). At the three sites with an unsaturated zone below the system (sites 2-4), no FC were ever observed in the 1-m downgradient wells. At the “substandard” site (site 1), where the bottom of the leaching structure extended below the water table, the geometric mean groundwater FC density at the 1-m well was 101.06 or 11 FC 100 mL-1 (n ) 6). However, after one additional meter of horizontal transport (2-m well), FC densities were attenuated to levels comparable with the other sites (Figure 6). No seasonal trends in effluent or groundwater densities were observed. The 4-5 log10 unit (>99.99%) density attenuation after only 1-2 m of subsurface transport at the study sites can be attributed to a combination of soil filtration, soil adsorption, and bacterial
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TABLE 2
Streamflow and Fecal Coliform (FC) Loads under Wet- and Dry-Weather Conditions, June 1-October 31, 1989 weather condition
period (d)
stream-flow (103 m3)
wet dry total
20 (13%) 133 (87%) 153
430 (17%) 2150 (83%) 2580
FC density (mean) (FC 100 mL-1)
FC load (total) (109 FC)
FC loading rate (mean) (109 FC d-1)
198 34
850 (54%) 731 (46%) 1580
43 5
The results suggest that groundwater flow, while a dominant component of the hydrologic cycle in this watershed and a major nutrient pathway to the bay (34), is of minor importance to FC transport in this study area. In other coastal regions of the United States, characterized by low-lying topography and fine-grained soils, breakout of effluent at the land surface is common, and septic systems have been shown to be a source of FC to coastal waters (5). In the present study area, the surficial geology is dominated by glacial outwash sands with a thick, well-oxygenated unsaturated zone unfavorable to the survival and transport of fecal coliforms (21). Further work is needed to assess the degree to which other, more resistant microbial agents (both viruses and bacteria) are transported through coastal watersheds of all types.
FIGURE 6. Fecal coliform densities in septic system effluent and groundwater downgradient from four septic systems. (b) Effluent (n ) 24) and groundwater 1 (9) and 2 m (2) downgradient of septic systems (6 monthly observations per system). Values are geometric means ( SE (- -) FC detection limit.
die-off and is consistent with the results of other studies (cf. ref 21). Detection of a more resistant bacterial indicator (Clostridium perfringens) in both the effluent and groundwater samples from all four sites (14) allowed us to reject the hypothesis that the wells detected no FC because they were mislocated with respect to possible soil macropores. Dilution with ambient groundwater accounted for less than 6% of the FC attenuation, as determined from parallel measurements of specific conductance in effluent and groundwater. The data are also consistent with the undetectable levels of FC found in water-table wells in the high-density residential portion of the watershed (14). Groundwater transport of FC in this study area appears to be relatively small, both on an areal and total watershed basis. Given the observed effluent FC densities (Figure 6) and previously reported water- and land-use data (34, 36), the 3088 building units overlying the watershed contribute about 1.25 × 1012 FC d-1 to the watershed subsurface. Even assuming a “worst case”, where the 400 septic systems closest to the bay and its tributaries contribute a fecal coliform load to adjacent surface waters equivalent to the worst case system studied after 1 m of subsurface transport (Figure 6), the load to the bay would be 7.7 × 106 FC d-1, or 5 orders of magnitude lower than the total load to the watershed. It should be noted that the number of substandard systems in the nearshore area is known to be far less than 400 and that an additional 70% decline in median FC density was observed during the second meter of horizontal transport at our substandard site (Figure 6). Nevertheless, this overestimate of the groundwater FC load is still only 0.14% of the other continuous, dry-weather source to bay waters, dry-weather streamflow (5.5 × 109 FC d-1).
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Bacterial Release from Protected Reservoirs. One process contributing to both dry- and wet-weather bacterial contamination of coastal embayments may be the release of previously deposited, animal-derived bacteria from protected microenvironments or reservoirs. On upland impermeable surfaces, animal feces themselves may serve as bacterial reservoirs (see above). Along the shoreline and on tidally-influenced reaches of the freshwater tributaries decaying aquatic vegetation (wrack) as well as sub- and intertidal sediments provide environments where moisture, temperature, light, and nutrient conditions favor the survival of enteric bacteria (8, 37). Measurements of FC release from wrack and resuspended sediment were made to test the potential importance of these reservoirs to FC contamination in Buttermilk Bay. Fecal coliform survival in wrack + bay-water suspensions versus seeded bay water controls indicated enhanced survival in the presence of wrack (Figure 7A), consistent with the large measured FC reservoir in beach wrack (see below). After an initial period of slight increase, bacterial densities in the wrack suspensions decreased about 1.0 log10 unit over a 30-day period compared to a large and rapid decline (3.5 log10 units in 4 days) in the bay water alone (Figure 7A). This finding suggests that enteric bacteria deposited by waterfowl and other wildlife in the shoreline wrack can survive not only from one high tide to the next but also from one spring-tide flooding event to the next (i.e., for over 2 weeks). When wrack samples were repeatedly rinsed with bay water, a continuous though decreasing elution of FC was observed (Figure 7B). The cumulative yield, while highly variable between samples, averaged 1.25 × 106 FC kg-1 of wrack (wet weight). We conclude that significant numbers of FC continue to be eluted from wrack deposits by repeated tidal flooding or precipitation events. Removal of wrack from a 100-m stretch of beach supported the hypothesis that wrack can be an important FC “source” in the field. In the week prior to wrack removal, water column FC densities in ebb tidal waters adjacent to the removal site ranged from 49 to 1600 FC 100 mL-1
TABLE 3
Release of Fecal Coliforms from Buttermilk Bay Sediments Due to Artificial Disturbance water column fecal coliform density (FC 100 mL-1) sampling date
sampling sitea
sediment type
before disturbance
after disturbanceb
3-5-86 3-5-86 3-5-86 3-5-86 3-5-86 3-5-86 8-29-86 8-29-86 8-29-86 9-11-86 9-11-86 9-11-86
A B F C D E B G E B G E
mud mud mud sand sand sand mud mud sand mud mud sand
