Septic Tank Density Research Articles

Characterization of Shiga toxin-producing Escherichia coli presence, serogroups and risk factors from private groundwater sources in western Ireland

Over recent years, Ireland has reported the highest crude incidence rates of Shiga toxin-producing Escherichia coli (STEC) enteritis in Europe. Unregulated private groundwater sources have emerged as an important potential transmission route for STEC, with up to 750,000 Irish residents reliant on these sources for domestic waters. This study aimed to investigate the prevalence and serogroup profile of STEC contamination from domestic private wells in western Ireland. Fifty-two groundwater sources were analysed during two sampling campaigns in the autumn (September/October) of 2019 (n = 21) and 2021 (n = 31). Untreated groundwater samples (30 L) were collected and analysed using the “CapE” (capture, amplify, extract) method. Extracted DNA was tested using multiplex real-time PCR for Shiga toxin stx1 and/or stx2 and eae genes. STEC positive DNA samples were tested for clinically relevant serogroups by real-time PCR. Data relating to 27 potential groundwater contamination risk factors were geospatially linked to each well and assessed for association with E. coli, stx1 and/or stx2 and eae presence/absence. Overall, 20/52 wells (38.4 %) were positive for E. coli (median concentration 8.5 MPN/100 mL as assessed by Colilert-18 method). Stx1 and/or stx2 was detected in 10/52 (19.2 %) wells overall and 8/20 E. coli positive wells, equating to a STEC to “generic” E. coli detection ratio of 40 %. Six of these wells (30 %) were also positive for eae. One or more serogroup-specific gene targets were identified in all but one stx1 and/or stx2 positive sample, with O145 (n = 6), O157 (n = 5) and O103 (n = 4) most prevalent. STEC presence was significantly associated with decreasing well depth (U = −2.243; p = 0.024) and increasing 30-day mean antecedent rainfall (U = 2.126; p = 0.034). Serogroup O104 was associated with increased sheep density (U = 2.089; p = 0.044) and detection of stx1 and/or stx2 + eae with increased septic tank density (U = 2.246 p = 0.023). Findings indicate high detection rates of clinically relevant STEC in E. coli contaminated groundwater sources in Ireland.

Connecting microbial, nutrient, physiochemical, and land use variables for the evaluation of water quality within mixed use watersheds

As non-point sources of pollution begin to overtake point sources in watersheds, source identification and complicating variables such as rainfall are growing in importance. Microbial source tracking (MST) allows for identification of fecal contamination sources in watersheds; when combined with data on land use and co-occuring variables (e.g., nutrients, sediment runoff) MST can provide a basis for understanding how to effectively remediate water quality. To determine spatial and temporal trends in microbial contamination and correlations between MST and nutrients, water samples (n = 136) were collected between April 2017 and May of 2018 during eight sampling events from 17 sites in 5 mixed-use watersheds. These samples were analyzed for three MST markers (human – B. theta; bovine – CowM2; porcine – Pig2Bac) along with E. coli, nutrients (nitrogen and phosphorus species), and physiochemical paramaters. These water quality variables were then paired with data on land use, streamflow, precipitation and management practices (e.g., tile drainage, septic tank density, tillage practices) to determine if any significant relationships existed between the observed microbial contamination and these variables. The porcine marker was the only marker that was highly correlated (p value <0.05) with nitrogen and phosphorus species in multiple clustering schemes. Significant relationships were also identified between MST markers and variables that demonstrated temporal trends driven by precipitation and spatial trends driven by septic tanks and management practices (tillage and drainage) when spatial clustering was employed.

Trends and influential factors of high ecological status mobility in Irish Rivers

The decline in high ecological water status in rivers is a significant concern in European countries. It is thus important to investigate the factors that cause sites to lose high status in order to undertake measures to protect and restore high status water quality. Analysis of 20 years of water quality data reveals strong mobility between high status and non-high status (especially good status) rivers. Associations between this mobility and socio-economic and physical environmental variables were estimated by multinomial logistic regression at national scale and regional scale. Based on reported changes in water quality status cross across 1990, 2000 and 2010, four classes of the mobility of high status were defined in this study: those sites that maintain high status (maintain), enter high status (enter), fluctuate between high and non-high status (fluctuate) and exit from high status (exit). The national results indicate that agricultural activity as indicated by variables representing intensity of livestock farming (organic nitrogen) and tillage farming (cereal share) and elevation had significant negative impacts on high status rivers. Meanwhile, significant differences in population density and septic tank density between ‘exit’, ‘maintain’, ‘fluctuate’ and ‘enter’ classes indicate that these factors played important roles in the stability of high status rivers. The regional outcomes reveal differential significant pressures across regions. For example, rainfall and elevation had positive impacts on high status rivers in the north-west region, while organic nitrogen had a negative effect in the south-west. This paper demonstrates the challenge in achieving the Water Framework Directive goal of maintaining high status rivers, given the sensitive and highly differentiated nature of areas that have lost high status or fluctuated in and out of high status. This paper also suggests the necessity for localised policies and mitigation measures.

Unraveling the ecological processes modulating the population structure of Escherichia coli in a highly polluted urban stream network

Escherichia coli dynamics in urban watersheds are affected by a complex balance among external inputs, niche modulation and genetic variability. To explore the ecological processes influencing E. coli spatial patterns, we analyzed its abundance and phylogenetic structure in water samples from a stream network with heterogeneous urban infrastructure and environmental conditions. Our results showed that environmental and infrastructure variables, such as macrophyte coverage, DIN and sewerage density, mostly explained E. coli abundance. Moreover, main generalist phylogroups A and B1 were found in high proportion, which, together with an observed negative relationship between E. coli abundance and phylogroup diversity, suggests that their dominance might be due to competitive exclusion. Lower frequency phylogroups were associated with sites of higher ecological disturbance, mainly involving simplified habitats, higher drainage infrastructure and septic tank density. In addition to the strong negative relationship between phylogroup diversity and dominance, the occurrence of these phylogroups would be associated with increased facilitated dispersal. Nutrients also contributed to explaining phylogroup distribution. Our study proposes the differential contribution of distinct ecological processes to the patterns of E. coli in an urban watershed, which is useful for the monitoring and management of fecal pollution.

Impact of the 2018 European drought on microbial groundwater quality in private domestic wells: A case study from a temperate maritime climate

A significant volume of research over the past two decades has highlighted both direct and indirect links between climate change and groundwater quality. However, to date, few studies have sought to explore the relationship(s) between drought conditions and groundwater quality in i) private (unregulated) groundwater sources, or ii) temperate maritime climates not commonly prone to drought events. The Republic of Ireland (ROI) represents an appropriate case-study due to its’ high reliance on private groundwater supplies, and while the region is largely unaffected by climatological extremes, modelling studies indicate that drier summers and drought conditions will increase in frequency. Accordingly, the current study sought to quantify the effects of the Summer 2018 drought experienced throughout Europe on private groundwater quality in the southwest of Ireland via an opportunistic field study. A repeated measures sampling campaign comprised of “drought” (June/July) and “post-drought” (October/November) analyses of 74 wells was undertaken, with complementary mapping and statistical analyses. Both Total Coliforms (TCs) and Escherichia coli (E. coli) were present during both drought (TCs: 42/74; 56.8%, E. coli: 7/74; 9.5%) and post-drought (TCs: 42/74; 56.8%, E. coli: 18/74; 24.3%) sampling periods. E. coli contamination during drought conditions was unexpected due to an absence of recharge or infiltration for microbial transport. Bivariate analyses suggest a hydrodynamic change, with the significance of E. coli sources and pathways shown to switch between sampling periods i.e. a shift from a combination of regional and local (site specific) contamination mechanisms, to solely site-specific mechanisms. More specifically, during drought conditions, septic tank density (p = 0.001) and local subsoil type (p = 0.009) were both associated with the presence of E. coli, while neither variable was significant during post-drought conditions. The current study is the first to provide a quantitative comparison of private groundwater quality during and after a large-scale drought event in a temperate maritime climate and may be used to improve our understanding of the effects of extreme events, and thus necessary preventative and monitoring strategies, going forward.

Sewage static system influence on the concentrations of nitrogen compounds in unconfined aquifers

ABSTRACT This research aimed to investigate the relation between sanitary situation and groundwater quality, using the concentration of nitrogenous compounds. The aquifer studied is unconfined and situated in the periurban zone of Fortaleza (NE Brazil). Through the Geographic Information System (GIS), a relational database was created using data from the IBGE demographic census (2011), to analyze numbers of households linked to septic tanks or rudimentary cesspit. The groundwater quality was evaluated based on nitrogen compounds (N-NH3 +; NO2 -; N-NO3 -), pH, and total dissolved solids (TDS). The highest concentrations of nitrates are found in areas with a higher density of septic tanks and rudimentary cesspit. Furthermore, nitrate was more present in water table above 6.6 m, mainly in the interfluvial zones, which have a high oxidation potential. The results contribute to the loss of contamination, based on the number of households with septic tanks and rudimentary cesspit, in unconfined aquifers, which were more vulnerable to contamination, mainly in peripheric expansions areas in the cities, where the deficit in sewage services tends to be high.

Discrimination of Nitrogen Sources in Karst Spring Contributing Areas Using a Bayesian Isotope Mixing Model and Wastewater Tracers (Florida, USA)

ABSTRACT Many springs in Florida have experienced a proliferation of nuisance algae and alteration of trophic structure in response to increases in nitrate concentration concurrent with rapid population growth and land use intensification beginning in the mid-20th century. While loading targets and remediation plans have been developed by state agencies to address excess nitrogen inputs, further confirmation of the relative contribution of nitrogen sources to groundwater is necessary to optimize the use of resources when implementing projects to reduce nitrogen loads. In the present study, stable isotopes of nitrate and wastewater indicators were used to discriminate sources of nitrogen in wells and springs in central Florida. Sampling was performed in 50 wells at 38 sites and at 10 springs with varying levels of agriculture and urban development. Nitrate isotope values were used to develop Bayesian mixing models to estimate the probability distribution of the contributions of nitrate sources in wells. Prior probabilities for the fractional contribution of each source were adjusted based on land use and density of septic tanks. Sucralose and the Cl:Br mass ratio were used as confirmatory indicators of wastewater sources. In residential areas, mixing model results indicated that fertilizer or mixed fertilizer and wastewater (septic tank effluent and reuse water) were the primary sources, with sucralose detections corresponding to wells with elevated contributions from wastewater. Sources of nitrogen in pasture and field crop areas were primarily fertilizer and manure; however, model posterior distributions of δ15N indicated that manure sources may have been overpredicted. The present study demonstrates the utility of a multi-tracer approach to build multiple lines of evidence to develop locally relevant remediation strategies for nitrogen sources in groundwater.

Determining the primary sources of fecal pollution using microbial source tracking assays combined with land-use information in the Edwards Aquifer

The Edwards Aquifer serves as a primary source of drinking water to more than 2 million people in south-central Texas, and as a karst aquifer, is vulnerable to human and animal fecal contamination which poses a serious risk to human and environmental health. A one-year study (Jan 2018 – Feb 2019) was conducted to determine the primary sources of fecal pollution along the Balcones and Leon Creek within the Edwards Aquifer recharge and contributing zones using general (E. coli, enterococci, and universal Bacteriodales) and host-associated (human-, dog-, cow- and chicken/duck-associated Bacteriodales) microbial source tracking (MST) assays. Additionally, sites were classified based on surrounding land use as a potential source predictor and marker levels were correlated with rain events and water quality parameters. Levels for the three general indicators were highest and exhibited similar trends across the sampling sites, suggesting that the sole use of these markers is not sufficient for specific fecal source identification. Among the host-associated markers, highest concentrations were observed for the dog marker (BacCan) in the Leon Creek area and the cow marker (BacCow) in the Balcones Creek area. Additionally, Chicken/Duck-Bac, BacCan and BacCow all exhibited higher concentrations during the spring season and the end of fall/early winter. Relatively lower concentrations were observed for the human-associated markers (HF183 and BacHum), however, levels were higher in the Leon Creek area and highest following rainfall events. Additionally, relatively higher levels in HF183 and BacHum were observed at sites having greater human population and septic tank density and may be attributed to leaks or breaks in these infrastructures. This study is the first to examine and compare fecal contamination at rural and urban areas in the recharge and contributing zones of the Edwards Aquifer using a molecular MST approach targeting Bacteroidales 16S rRNA gene-based assays. The Bacteroidales marker assays, when combined with land use and weather information, can allow for a better understanding of the sources and fluxes of fecal contamination, which can help devise effective mitigation measures to protect water quality.

An integrative GIS approach to analyzing the impacts of septic systems on the coast of Florida, USA

ABSTRACT An estimated 2.7 million septic systems in Florida, USA are potential ground and surface water contaminant sources that may affect environmental and human health. This study examined the spatial distribution of septic systems, coastal surface water contamination, and related environmental factors of coastal Florida watersheds at the 8-digit hydrologic unit code level. Hydrology, in situ sampling data, and other ancillary data were combined in a geographic information system to examine spatial relationships. Spatial distribution data were correlated to nitrogen, Enterococci counts, and beach closures tabulated since 2000, 2007, and 2012. Significant positive correlations (α = 0.05) with nitrogen and Enterococci counts were consistent for percent agricultural cover, percent combined urban and agricultural cover, septic tank density, population density, and septic tank density in poorly drained soils. Beach closures since 2012 were significantly positively correlated (α = 0.05) to average impervious cover (IC) and percent urbanization. Statistics indicated that Enterococci counts, nitrogen, and beach closures may be related to specific environmental factors and septic tank densities. The combination of septic tanks in urban regions with high IC prone to elevated runoff could also be a factor in surface water contamination. Data availability was also highlighted as a limitation due to infrequent spatial and temporal sampling.

Exploring private water wells for fecal sources and evidence of pathogen presence in the context of current testing practices for potability in Ontario

Abstract Private water wells provide drinking water for an estimated 4.1 million households in Canada yet remain understudied in the context of microbial water quality or human health impacts. As there exists little systematic surveillance for enteric infections or outbreaks related to well water sources, consumers may be at risk of waterborne infectious diseases. A standard protocol in Ontario requires 200 mL of water, collected, and submitted by well owners, half of which is used to analyze for Escherichia coli and total coliforms (TCs). The aim of this study was to determine the efficacy of testing small water volumes and to survey for other contaminants in addition to bacterial indicators to inform pathogen prevalence and fecal source in drinking water wells. Samples were assessed for E. coli and TCs, by culture, and genetic markers of Bacteroides spp., Campylobacter spp., Salmonella spp., and Shiga toxin-producing E. coli, using qPCR. The source of fecal contamination varied by the geographic region and may be explained by septic tank density and underlying geology, among other factors. A small number of samples (1.9%) showed the evidence of contamination with enteric pathogens. Lastly, E. coli measured by qPCR, as opposed to culture, correlated more strongly to Bacteroides markers.

Evaluation of levels of antibiotic resistance in groundwater-derived E. coli isolates in the Midwest of Ireland and elucidation of potential predictors of resistance

Antibiotic-resistant (pathogenic and non-pathogenic) organisms and genes are now acknowledged as significant emerging aquatic contaminants with potentially adverse human and ecological health impacts, and thus require monitoring. This study is the first to investigate levels of resistance among Irish groundwater (private wells) samples; Escherichia coli isolates were examined against a panel of commonly prescribed human and veterinary therapeutic antibiotics, followed by determination of the causative factors of resistance. Overall, 42 confirmed E. coli isolates were recovered from a groundwater-sampling cohort. Resistance to the human panel of antibiotics was moderate; nine (21.4%) E. coli isolates demonstrated resistance to one or more human antibiotics. Conversely, extremely high levels of resistance to veterinary antibiotics were found, with all isolates presenting resistance to one or more veterinary antibiotics. Particularly high levels of resistance (93%) were found with respect to the aminoglycoside class of antibiotics. Results of statistical analysis indicate a significant association between the presence of human (multiple) antibiotic resistance (p = 0.002–0.011) and both septic tank density and the presence of vulnerable sub-populations (<5 years). For the veterinary antibiotics, results point to a significant relationship (p = <0.001) between livestock (cattle) density and the prevalence of multiple antibiotic resistant E. coli. Groundwater continues to be an important resource in Ireland, particularly in rural areas; thus, results of this preliminary study offer a valuable insight into the prevalence of antibiotic resistance in the hydrogeological environment and establish a need for further research with a larger geological diversity.

Patterns of Wastewater Infrastructure along a Gradient of Coastal Urbanization: A Study of the Puget Sound Region

The aim of this paper is to explore patterns of wastewater infrastructures (sewers vs. septic tanks) in urbanizing watersheds across a coastal metropolitan region. This research combines an urban-rural gradient with spatial metrics at the patch and watershed scale (proportion of parcels on a treatment system, septic density, lot size and percent imperviousness) to analyze wastewater patterns in the Puget Sound, WA, USA. Results show that most urban residential parcels are hooked up to a sewer, although there remain urban residences on a septic tank with small lots. I find a complex arrangement of wastewater treatment in suburban watersheds representing a patchwork of parcels on sewers and septic tanks. Sewers dominate in total numbers, while the density of septic tanks is highest in this portion of the urban gradient. Lot size decreases from rural to urban; however, it varies depending on the type of wastewater treatment system. In urban watersheds, lots on septic tanks are significantly smaller than lots in suburban and rural watersheds and of a similar size compared to lots on sewers. I also find a significant difference in the amount of impervious surfaces in watersheds dominated by sewers vs. septic tanks. In the urban portion of the gradient, the amount of paved surfaces in parcels with septic tanks is also similar in level as parcels with sewers. I discuss how these patterns emerge from the interplay of biophysical, socio-economic and technological factors and how different regulatory regimes for septic tanks and sewers may further induce these patterns.

Determining Hot Spots of Fecal Contamination in a Tropical Watershed by Combining Land-Use Information and Meteorological Data with Source-Specific Assays

The objective of this study was to combine knowledge of environmental, topographical, meteorological, and anthropologic factors in the Río Grande de Arecibo (RGA) watershed in Puerto Rico with information provided by microbial source tracking (MST) to map hot spots (i.e., likely sources) of fecal contamination. Water samples were tested for the presence of human and bovine fecal contamination in addition to fecal indicator bacteria and correlated against several land uses and the density of septic tanks, sewers, and latrines. Specifically, human sources were positively correlated with developed (r = 0.68), barren land uses (r = 0.84), density of septic tanks (r = 0.78), slope (r = 0.63), and the proximity to wastewater treatment plants (WWTPs) (r = 0.82). Agricultural land, the number of upstream National Pollution Discharge Elimination System (NPDES) facilities, and density of latrines were positively associated with the bovine marker (r = 0.71; r = 0.74; and r = 0.68, respectively). Using this information, we provided a hot spot map, which shows areas that should be closely monitored for fecal contamination in the RGA watershed. The results indicated that additional bovine assays are needed in tropical regions. We concluded that meteorological, topographical, anthropogenic, and land cover data are needed to evaluate and verify the performance of MST assays and, therefore, to identify important sources of fecal contamination in environmental waters.

Relationships between stream phosphorus concentrations and drainage basin characteristics in a watershed with poultry farming

In the Illinois River watershed (Arkansas and Oklahoma) poultry litter is routinely applied to pasture lands as fertilizer supplements. The objective of this study was to quantify the impacts of this practice on water quality in small streams in the watershed. Total phosphorus concentrations in small streams were quantified during a 15 month intensive sampling program for small tributaries, un-impacted by point sources, and covering a range of upstream poultry farming intensities throughout the watershed. Base flow samples were also collected from the tributaries during multiple dry weather sampling events. Correlation analysis was performed for various measurable drainage basin characteristics and phosphorus concentrations. Results show significant positive correlations between total phosphorus concentrations and the intensity of poultry farm operations upstream of sample sites. This relationship is supported by a supplemental synoptic sampling event upstream and downstream of a field applied with poultry litter and by a watershed-scale mass balance analysis. The latter identifies poultry as the single largest contributor of phosphorus inputs to the basin, accounting for over 76 % of the total watershed load. Study results also show significant, but generally weaker, positive correlations between phosphorus and upstream density of septic tanks and soil conservation service curve number of the drainage basin, but these appear likely to be primarily due to cross-correlations with poultry farm presence. This study demonstrates the use of readily-observed basin characteristics in simple water quality predictive tools and has implications for management of poultry-dominated watersheds.

Urban Stormwater Runoff Drives Denitrifying Community Composition Through Changes in Sediment Texture and Carbon Content

The export of nitrogen from urban catchments is a global problem, and denitrifying bacteria in stream ecosystems are critical for reducing in-stream N. However, the environmental factors that control the composition of denitrifying communities in streams are not well understood. We determined whether denitrifying community composition in sediments of nine streams on the eastern fringe of Melbourne, Australia was correlated with two measures of catchment urban impact: effective imperviousness (EI, the proportion of a catchment covered by impervious surfaces with direct connection to streams) or septic tank density (which affects stream water chemistry, particularly stream N concentrations). Denitrifying community structure was examined by comparing terminal restriction fragment length polymorphisms of nosZ genes in the sediments, as the nosZ gene codes for nitrous oxide reductase, the last step in the denitrification pathway. We also determined the chemical and physical characteristics of the streams that were best correlated with denitrifying community composition. EI was strongly correlated with community composition and sediment physical and chemical properties, while septic tank density was not. Sites with high EI were sandier, with less fine sediment and lower organic carbon content, higher sediment cations (calcium, sodium and magnesium) and water filterable reactive phosphorus concentrations. These were also the best small-scale environmental variables that explained denitrifying community composition. Among our study streams, which differed in the degree of urban stormwater impact, sediment grain size and carbon content are the most likely drivers of change in community composition. Denitrifying community composition is another in a long list of ecological indicators that suggest the profound degradation of streams is caused by urban stormwater runoff. While the relationships between denitrifying community composition and denitrification rates are yet to be unequivocally established, landscape-scale indices of environmental impact such as EI may prove to be useful indicators of change in microbial communities.

The importance of upland flow paths in determining urban effects on stream ecosystems

AbstractMitigation of urban effects on streams requires an understanding of the paths by which urban effects are transmitted from catchments to streams and how those effects are attenuated with distance. We assessed whether modeling attenuation from impervious surfaces and septic tanks along drainage lines improved prediction of 3 instream ecological indicators. Eleven regression models were calculated for each indicator (Escherichia coli, NO3/NO2, and Stream Invertebrate Grade Number Average Level [SIGNAL; a macroinvertebrate assemblage composition index]). Predictor variables included imperviousness or septic-tank density with no attenuation (i.e., total imperviousness and tank density), with overland attenuation (exponential decay with distance along topographic flow paths to stormwater drain or to stream), and with overland and instream attenuation (exponential decay with distance travelled within the stream). Escherichia coli was best predicted by the weighted density of septic tanks, with their infl...

The influence of urban density and drainage infrastructure on the concentrations and loads of pollutants in small streams.

Effective water quality management of streams in urbanized basins requires identification of the elements of urbanization that contribute most to pollutant concentrations and loads. Drainage connection (the proportion of impervious area directly connected to streams by pipes or lined drains) is proposed as a variable explaining variance in the generally weak relationships between pollutant concentrations and imperviousness. Fifteen small streams draining independent subbasins east of Melbourne, Australia, were sampled for a suite of water quality variables. Geometric mean concentrations of all variables were calculated separately for baseflow and storm events, and these, together with estimates of runoff derived from a rainfall-runoff model, were used to estimate mean annual loads. Patterns of concentrations among the streams were assessed against patterns of imperviousness, drainage connection, unsealed (unpaved) road density, elevation, longitude (all of which were intercorrelated), septic tank density, and basin area. Baseflow and storm event concentrations of dissolved organic carbon (DOC), filterable reactive phosphorus (FRP), total phosphorus (TP) and ammonium, along with electrical conductivity (EC), all increased with imperviousness and its correlates. Hierarchical partitioning showed that DOC, EC, FRP, and storm event TP were independently correlated with drainage connection more strongly than could be explained by chance. Neither pH nor total suspended solids concentrations were strongly correlated with any basin variable. Oxidized and total nitrogen concentrations were most strongly explained by septic tank density. Loads of all variables were strongly correlated with imperviousness and connection. Priority should be given to low-impact urban design, which primarily involves reducing drainage connection, to minimize urbanization-related pollutant impacts on streams.

Catchment urbanisation and increased benthic algal biomass in streams: linking mechanisms to management

Summary1. Urbanisation is an important cause of eutrophication in waters draining urban areas. We determined whether benthic algal biomass in small streams draining urban areas was explained primarily by small‐scale factors (benthic light, substratum type and nutrient concentrations) within a stream, or by catchment‐scale variables that incorporate the interacting multiple impacts of urbanisation (i.e. variables that describe urban density and the intensity of drainage or septic tank systems).2. Benthic algal biomass was assessed as chlorophyll a density (chl a) in 16 streams spanning a rural–urban gradient, with both a wide range of urban density and of piped stormwater infrastructure intensity on the eastern fringe of metropolitan Melbourne, Australia. The gradient of urban density among streams was broadly correlated with catchment imperviousness, drainage connection (proportion of impervious areas connected to streams by stormwater pipes), altitude, longitude and median phosphorus concentration. Catchment area, septic tank density, median nitrogen concentration, benthic light (photosynthetically active radiation) and substratum type were not strongly correlated with the urban gradient.3. Variation in benthic light and substratum type within streams explained a relatively small amount of variation in log chl a (3–11 and 1–13%, respectively) compared with between‐site variation (39–54%).4. Median chl a was positively correlated with catchment urbanisation, with a large proportion of variance explained jointly (as determined by hierarchical partitioning) by those variables correlated with urban density. Independent of this correlation, the contributions of drainage connection and altitude to the explained variance in chl a were significant.5. The direct connection of impervious surfaces to streams by stormwater pipes is hypothesised as the main determinant of algal biomass in these streams through its effect on the supply of phosphorus, possibly in interaction with stormwater‐related impacts on grazing fauna. Management of benthic algal biomass in streams of urbanised catchments is likely to be most effective through the application of stormwater management approaches that reduce drainage connection.

Stormwater drainage pipes as a threat to a stream-dwelling amphipod of conservation significance, Austrogammarus australis, in southeastern Australia

The general hypothesis that catchment urbanization explained the distribution of the threatened, stream-dwelling amphipod Austrogammarus australis (listed under the Victorian Flora and Fauna Guarantee Act 1988) was tested using several surveys of 58 sites in streams draining the Dandenong Ranges on the eastern fringe of Melbourne, Victoria, Australia. More specifically, four catchment-scale elements of urban land, hypothesized as sources of stress to receiving streams, were separated: catchment imperviousness, drainage connection (proportion of impervious areas connected to streams by stormwater pipes), density of unsealed roads and density of septic tanks. The degree to which each attribute independently and jointly explained the occurrence of A. australis was assessed using hierarchical partitioning of logistic regression analyses. Drainage connection independently best explained the occurrence of A. australis, pointing to stormwater drainage design as the priority area of catchment management for the conservation of the species. The separation of urban land use into attributes that characterize likely stressor sources provides a useful framework for assessing and prioritizing the most appropriate management actions to minimize urban-related stresses to aquatic biota.

Septic Tank Density and Ground‐Water Contamination

ABSTRACTAs more and more cases of ground‐water contamination are reported, the public has become increasingly aware of the importance of preserving the quality of this limited resource, especially in areas totally dependent on ground‐water sources. Although most of the attention is focused on pollution by organic chemicals, these compounds are responsible for a relatively small percentage of ground‐water‐related disease outbreaks. The majority of waterborne disease outbreaks are caused by bacteria and viruses present in domestic sewage. Septic tanks contribute the largest volume of waste water, 800 billion gallons per year to the subsurface, and are the most frequently reported cause of ground‐water contamination associated with disease outbreaks. The U.S. Environmental Protection Agency has designated areas with septic tank densities of greater than 40 systems per mi2 (1 system per 16 acres) as regions of potential ground‐water contamination. Numerous cases of ground‐water contamination have been reported in areas of high septic tank density; lot sizes in these areas range from less than one‐quarter acre to three acres. The single most important means of limiting ground‐water contamination by septic tanks is to restrict the density of these systems in an area.