Mike H. Barrett

Risk factors contributing to microbiological contamination of shallow groundwater in Kampala, Uganda

A study of water quality variation in shallow protected springs in Kampala was undertaken over a 12-month period to assess the causes of microbiological contamination. The microbiological quality of water was assessed using thermotolerant coliforms and faecal streptococci. Sanitary inspections and hazard assessments were undertaken to identify faecal sources (hazards), contaminant pathways and contributory factors. Data were collected on rainfall and population as additional factors potentially exerting an influence on microbiological quality. Initial analysis of the data showed a significant relationship between median level of contamination and rainfall, in particular to short-term rainfall events. Total sanitary risk score showed a significant relationship with median level of contamination, but population density may be a confounding factor. The raw microbiological data were transformed into five water quality targets: <1 and < or =10 cfu 100ml(-1) for faecal streptococci; and <1, < or =10 and < or=50 cfu 100 ml(-1) for thermotolerant coliforms. The presence of individual risk factors as well as variables for rainfall and population density were analysed with respect to failure to meet these water quality targets using contingency tables. Logistic regression models were developed for each of the five water quality targets. The analysis strongly suggested that there is rapid recharge of the springs after rainfall and this leads to microbiological contamination. On-site sanitation was less important than other sources of faecal matter, which was consistent with a low sanitation coverage in the study area. The study suggested that improving sanitary completion and local environmental hygiene was more important than controlling on-site sanitation in improving the quality of these springs.

Marker species for identifying urban groundwater recharge sources: A review and case study in Nottingham, UK

Abstract Urban environments significantly alter the nature of recharge to underlying aquifers. Direct precipitation is reduced, but additional recharge may result from storm water runoff, mains supply leakage and sewer leakage. If urban aquifers are to be effectively and sustainably managed, it is vital that these recharge sources should be identified and quantified. A sound theoretical approach is the use of marker species for identifying the three principal sources of urban recharge (precipitation, mains and sewers). The ideal marker species should be unique to a particular recharge source (irrespective of geographic location), and easily identifiable in the groundwater system, enabling quantification of that source. A review of potential markers and a detailed study of the aquifer beneath the city of Nottingham, UK, was unable to find suitable markers for precipitation and mains leakage. Trihalomethanes, which are chlorination by-products, and so a potential marker of mains water, were hardly detected in either mains or groundwater. More potential markers are available for sewage, including d-limonene, which is a new ingredient in some detergents. For shallow groundwater, the most effective means of identifying sewage recharge was a combination of stable nitrogen isotopes and microbiological indicators; effectively a sewage “fingerprint”. This study confirms the need for a multi-component approach rather than using individual marker species. Additionally it demonstrates that the impact of sewer leakage on groundwater quality beneath Nottingham is generally not high.

Microbial contamination of two urban sandstone aquifers in the UK

Development of urban groundwater has historically been constrained by concerns about its quality. Rising urban water tables and overabstraction from rural aquifers in the UK have led to a renewed interest in urban groundwater, particularly the possibility of finding water of acceptable quality at depth. This study assessed the microbial quality of groundwater collected from depth-specific intervals over a 15-month period within the Permo-Triassic Sherwood Sandstone aquifers underlying the cities of Nottingham and Birmingham. Sewage-derived bacteria (thermotolerant coliforms, faecal streptococci and sulphite-reducing clostridia) and viruses (enteroviruses, Norwalk-like viruses, coliphage) were regularly detected to depths of 60 m in the unconfined sandstone and to a depth of 91 m in the confined sandstone. Microbial concentrations varied temporally and spatially but increased frequency of contamination with depth coincided with geological heterogeneities such as fissures and mudstone bands. Significantly, detection of Norwalk-like viruses and Coxsackievirus B4 in groundwater corresponded with seasonal variations in virus discharge to the sewer system. The observation of low levels of sewage-derived microbial contaminants at depth in the Triassic Sandstone aquifer is explained by the movement of infinitesimal proportions of bulk (macroscopic) groundwater flow along preferential pathways (e.g., fissures, bedding planes). The existence of very high microbial populations at source (raw sewage) and their extremely low detection limits at the receptor (multilevel piezometer) enable these statistically extreme (microscopic) flows to be traced. Rapid penetration of microbial contaminants into sandstone aquifers, not previously reported, highlights the vulnerability of sandstone aquifers to microbial contamination.

Vertical groundwater flow in Permo-Triassic sediments underlying two cities in the Trent River Basin (UK)

The vertical component of groundwater flow that is responsible for advective penetration of contaminants in sandstone aquifers is poorly understood. This lack of knowledge is of particular concern in urban areas where abstraction disrupts natural groundwater flow regimes and there exists an increased density of contaminant sources. Vertical hydraulic gradients that control vertical groundwater flow were investigated using bundled multilevel piezometers and a double-packer assembly in dedicated boreholes constructed to depths of between 50 and 92 m below ground level in Permo-Triassic sediments underlying two cities within the Trent River Basin of central England (Birmingham, Nottingham). The hydrostratigraphy of the PermoTriassic sediments, indicated by geophysical logging and hydraulic (packer) testing, demonstrates considerable control over observed vertical hydraulic gradients and, hence, vertical groundwater flow. The direction and magnitude of vertical hydraulic gradients recorded in multilevel piezometers and packers are broadly complementary and range, within error, from þ0.1 to 2 0.7. Groundwater is generally found to flow vertically toward transmissive zones within the hydrostratigraphical profile though urban abstraction from the Sherwood Sandstone aquifer also influences observed vertical hydraulic gradients. Bulk, downward Darcy velocities at two locations affected by abstraction are estimated to be in the order of several metres per year. Consistency in the distribution of hydraulic head with depth in Permo-Triassic sediments is observed over a one-year period and adds support the deduction of hydrostratigraphic control over vertical groundwater flow. q 2003 Elsevier B.V. All rights reserved.

On-site sanitation and urban aquifer systems in Uganda

Absence of microbial indicators from groundwater does not always mean absence of contamination. Research in sub-Saharan Africa warns against generalized assumptions and solutions.

A continuous flow method for calculating adsorption isotherms of organic pollutants onto aquifer materials

Abstract A continuous flow method for the construction of adsorption isotherms has been developed. The method is semi-automated and has a computer-controlled data collection and storage process. The technique produces high-accuracy isotherms based on several hundred data points. Results have been produced for several mineral-pollutant systems. These have shown significant adsorption may occur even when using low-organic-carbon sorbents, the degree to which the adsorption occurs increasing with decreasing sorbate solubility.