Loading and mobility of faecal and chemical contaminants to shallow groundwater: evidence from Lukaya Town, Central Uganda

Abstract

Shallow groundwater and on-site sanitation systems (OSS) feature centrally in most low income countries pursuing United Nations’ Sustainable Development Goal 6 (SDG 6) by 2030. This study assessed the shallow hydrogeological and physical environments which are used conjunctively to sustain supply of water and repository of faecal matter in Lukaya town in Uganda from 2018 to 2019. This was due to limited information on hydrogeological and dominant factors influencing faecal contaminant mobility to shallow groundwater. Shallow groundwater is obtained from weathered basement rocks as well as alluvial and lacustrine sediments with aquifer thicknesses of 2 to 10 m, transmissivity (T) and hydraulic conductivity (K) values ranging from ~ 4 to 40 m2 /day and 0.6 to 1.6 m/day, respectively, recharged by precipitation. Average rainfall is 839 mm/year with estimated recharge of up to 22 in December (2018) and 27 mm/month in May (2019) during two wet seasons. Faecal indicators (Escherichia coli and NO3) originating from the loading of faecal contaminants in the subsurface is by the use of OSS including pit latrines and septic tanks, reveal that shallow groundwater is vulnerable to contamination. Using a t-test p-value < 0.05, higher mean (± standard deviation) E. coli counts are recorded during the dry season (45±80 x 103cfu/100 mL) compared to the wet season (2±6 x 103 cfu/100 mL). Monthly NO3 concentrations varied spatially but remained largely stable during dry (17±32 mg/L) and wet (22±21 mg/L) seasons. Nitrate was found to be an unreliable indicator of seasonal variations in faecal contamination as nitrification is impeded by rapid infiltration of direct groundwater recharge inferred from groundwater-level responses to heavy (>10 mm/day) rainfall events. Other monthly physicochemical parameters such as electrical conductivity (mean: 172±117 µS/cm) and temperature (mean: 24.7±0.9 oC) similarly remained stable over the year; pH was slightly lower during the wet season (5.2±0.6) compared to the dry (5.4±0.4). The mobility of faecal and chemical indicators of contamination was assessed by analysing the quality of water and the factors which influence the transmission of contaminants to shallow groundwater systems. A statistically significant (p = 0.004) multiple linear regression model from dry-season E. coli counts in 2018 identifies inadequacies of construction and maintenance as key factors (p = 0.006) for faecal contamination. Other factors were: distance to the nearest OSS, source type (shallow), risk level (medium) with p values of 0.035, 0.026, 0.003, respectively. As shallow groundwater and on-site sanitation systems are primary sources of domestic water and repositories of faecal matter in Lukaya Town, greater support is required to improve the functionality and sustainability of these on-site systems to realise UN SDG 6.