Unsaturated zone flow processes and aquifer response time in the Chalk Aquifer, Brighton, South East England

The Chalk aquifer is one of the main sources of water in South East England. The unsaturated zone in the aquifer plays animportant role controlling the time and magnitude of recharge and is major pathway for contaminant transport to the water table.A range of previous work has addressed flow processes in the Chalk unsaturated zone, but physical understanding is still incomplete.Here we present the results of a study on flow mechanism in the Chalk unsaturated zone using a combination of statistical analysisand novel laboratory methods. The study was undertaken at three sites (North Heath Barn [NHB], Pyecombe East [PE], and PrestonPark [PP]) on the Chalk of the Brighton block, South East England. Daily and hourly time series data of groundwater level andrainfall were correlated. The results show that a slower groundwater level response to rainfall occurs during dry seasons (summerand autumn) when the amount of effective rainfall is less than 4 mm/day, with a thicker and drier unsaturated zone. A fasterresponse occurs during wet seasons (winter and spring) when the daily effective rainfall exceeds 4 mm/day with a thinner andwetter unsaturated zone. Periods of very rapid response (within 15 h) were observed during wet seasons at NHB and PE sites, withunsaturated hydraulic conductivity (Ku) inferred to reach 839 mm/day. A slower response was observed at an urbanized site (PP) asa result of reduction in direct recharge due to reduced infiltration, due to presences of impermeable infrastructure covering the areaaround PP borehole. Laboratory measurements of Ku of the Chalk matrix using a geotechnical centrifuge show variation from 4.27to 0.07 mm/day, according to the level of saturation. Thus, the rapid responses cannot be linked to matrix flow only but indicate thecontribution of fracture and karstic flow processes in conducting water