Urban groundwater baseflow influence upon inorganic river-water quality: The River Tame headwaters catchment in the City of Birmingham, UK

SummaryUnderstanding the linkage between urban land, groundwater, baseflow and river contamination at the city scale is lacking. This study evaluates the influence of inorganic (major/minor ions and metals) groundwater contamination in the Triassic sandstone–Quaternary deposits aquifer system underlying the City of Birmingham, UK upon the baseflow and water quality of the river Tame. Baseflow water-quality data have been collected from a riverbed piezometer network installed in the 7.4 km reach crossing the effluent unconfined sandstone aquifer and compared to river and aquifer water-quality data. Overall, the inorganic chemical quality of the baseflow was not as poor as potentially surmised from the urbanisation present. Baseflow impact upon river-water quality was also low. These conclusions were underpinned by evidences of: limited river-water quality changes along the reach; some river concentrations being diluted by better quality baseflow; only occasional breaching of water-quality criteria; limited impact upon river-reach quality local to elevated baseflow dicharges; natural attenuation occurrence within the riverbed; and, modest, albeit somewhat uncertain, baseflow mass fluxes. Baseflow fluxes to the reach were in the ranges 100–3500 t/yr for major ions, 1–50 t/yr for minor ions and 1–500 kg/yr for toxic metals with zinc and nickel most prominent. The sporadic occurrence of elevated baseflow concentrations was ascribed to discrete groundwater plume discharges. More detailed sub-reach studies would be required to fully resolve discrete plume baseflow contributions and improve mass flux estimates. Not uncommonly, the urban river studied was already contaminated and hence persistent baseflow fluxes may assume more importance if the river became cleaner through other control measures. Future research should hence consider the emergent significance of urban baseflows. There are needs to: conduct similar studies to investigate if city-scale baseflow impacts are comparable elsewhere; research the importance of spatially and temporally dynamic attenuation in the riverbed; and, develop improved baseflow mass flux estimation methods.

Research highlights
► City influence on inorganic chemical quality of groundwater baseflow is studied.
► Birmingham dataset of urban groundwater, baseflow and river quality are compared.
► Baseflow quality is not as poor as perhaps surmised from the urbanisation present.
► Discrete groundwater plume discharges sporadically observed over a 7 km reach.
► Baseflow contaminant mass fluxes had limited impact on urban river Tame quality.