Assessing "Urban Karst" Effects from Groundwater-Storm Sewer System Interaction in a Till Aquitard.

Storm sewer systems and their associated utility trenches may strongly influence the effects of urbanization on a groundwater system. This study was undertaken to identify the causes of district-wide basement infiltration in an aquitard system. It comprised widespread continuous monitoring of utility trench wells and dye tracing from storm sewer system exfiltration tests. The results indicate that a major effect of urbanization on shallow groundwater is related to storm sewer system exfiltration, which is marked by a characteristic pattern of head variations in the aquitard unrelated to distributed surface infiltration. The aquitard constrains flow from storm sewer system exfiltration to the utility trench, creating an urban flow path for groundwater discharge. Temporary buildup of water levels in the utility trench drives relatively high-velocity flow through the permeable sewer bedding material of the utility trench to a separate foundation drainage collector system, ultimately causing a severe "urban karst" effect that produces system surcharging and widespread basement water infiltration. The main conditions causing the "urban karst" are the large hydraulic conductivity ratio between the utility trench material and the aquitard, and the shallow depth and low gradient of the storm sewer system imposed by a very flat drainage basin.

Highway deicing salt dynamic runoff to surface water and subsequent infiltration to groundwater during severe UK winters.

Dynamic impact to the water environment of deicing salt application at a major highway (motorway) interchange in the UK is quantitatively evaluated for two recent severe UK winters. The contaminant transport pathway studied allowed controls on dynamic highway runoff and storm-sewer discharge to a receiving stream and its subsequent leakage to an underlying sandstone aquifer, including possible contribution to long-term chloride increases in supply wells, to be evaluated. Logged stream electrical-conductivity (EC) to estimate chloride concentrations, stream flow, climate and motorway salt application data were used to assess salt fate. Stream loading was responsive to salt applications and climate variability influencing salt release. Chloride (via EC) was predicted to exceed the stream Environmental Quality Standard (250mg/l) for 33% and 18% of the two winters. Maximum stream concentrations (3500mg/l, 15% sea water salinity) were ascribed to salt-induced melting and drainage of highway snowfall without dilution from, still frozen, catchment water. Salt persistance on the highway under dry-cold conditions was inferred from stream observations of delayed salt removal. Streambed and stream-loss data demonstrated chloride infiltration could occur to the underlying aquifer with mild and severe winter stream leakage estimated to account for 21 to 54% respectively of the 70t of increased chloride (over baseline) annually abstracted by supply wells. Deicing salt infiltration lateral to the highway alongside other urban/natural sources were inferred to contribute the shortfall. Challenges in quantifying chloride mass/fluxes (flow gauge accuracy at high flows, salt loading from other roads, weaker chloride-EC correlation at low concentrations), may be largely overcome by modest investment in enhanced data acquisition or minor approach modification. The increased understanding of deicing salt dynamic loading to the water environment obtained is relevant to improved groundwater resource management, highway salt application practice, surface-water - ecosystem management, and decision making on highway drainage to ground.