from La Plata University (Argentina). He joined XR Consultants and worked as a field and structural geologist in the Subandes from 1998 to 2000. He spent two years as a visiting scientist at Cornell University working on the Subandes stratigraphy and structural geology. Now, he is a postdoctoral research associate at Colorado School of Mines.

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Magnetostratigraphy, isotopic dating, and sandstone petrography establish age limits on the depositional history of ϳ2100 m of foreland basin strata in the Neogene Metán Subgroup of northwest Argentina. The strata were deposited between ca. 15.1 and 9.7 Ma in the eastern Sistema de Santa Bárbara. The region is positioned above the Cretaceous Salta rift basin, in the Transition Zone between modern relatively steep and flat subducting segments of the Nazca plate.Formations within the subgroup are shown to be diachronous over a 60 km distance; the younger ages are in the east. Changes in paleocurrent flow directions and the lithic clast component of sandstones collected from the Arroyo González section suggest that basal fluvial strata were derived from the craton to the east beginning in middle Miocene time, just prior to 15.1 Ma. By ca. 14.5 Ma, the paleocurrent flowed from a source in the west and sedi-*Second address: Magstrat, LLC, ment accumulation rates increased dramatically. These changes correlate with contemporaneous tectonism in the west. A local increase in basin accommodation may be partly related to a zone of weakness near the eastern boundary of the Salta rift.Uplift in the western Cordillera Oriental apparently began by 13.7 Ma and thrusting rapidly migrated eastward. The eastern Cordillera Oriental ranges began to rise between 25؇ and 26؇S ca. 10 Ma. As thrusting migrated eastward, low-energy depositional environments were overwhelmed ca. 13.7 Ma. Above an erosional unconformity that removed strata to an age of ca. 9.7 Ma, basal strata from the overlying Jujuy Subgroup were deposited beginning after 9 Ma.Sandstones from Río Yacones suggest that the Cordillera Oriental uplift continued for several million years longer between 24؇ and 25؇S. Uplift of the Sistema de Santa Bárbara, in the distal portion of the foreland, did not begin until after ca. 9 Ma.

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INTRODUCTIONUnlike many developing countries, and even many parts of North America, on-site disposal of sewage is rarely practised in the UK, with over 94% of the population being served by a sewerage system(l). Such a system comprises a length of conduit which, because of faults, might allow sewage to exfiltrate into the surrounding ground and eventually into groundwater. Concern over the adverse affect that this leakage might cause on groundwater quality has been expressed for many year^'^,^^'; however, subsequent research indicated that pollution was restricted to the immediate vicinity of the sewer and did not travel to the deeper aquifers. This research was used by the industry Regulator, who stated that 'sewage exfiltration is mainly biological in nature, and bacteria in the ground would render it harmless before it could reach the groundwater table and cause any p~llution'~? Therefore, the prevention of leakage from sewers is not considered by the Regulator when setting water-industry performance or financial targets. These findings were based on passive monitoring studies, using existing groundwaterabstraction points which are only likely to reveal extensive contamination. More recent studies using active monitoring (purposeinstalled monitoring points around sewers) reveal contamination of the shallow subsurface and shallow groundwater from leaking sewers to be widespread and not necessarily related to the age of the sewerage system"). There is, therefore, a need to understand why sewers leak, to quantify the extent of sewage exfiltration and the effect that this has on the groundwater resource. ABSTRACTExfiltration (of sewage into the ground from faults in ageing sewerage networks) has been recognised for many years but has never been quantified. lt was considered that this leakage did not pose a groundwater-contamination threat because it was assumed that pathogens in sewage would either die off or be attenuated before reaching the groundwater level. This paper identifies why sewers have faults that could allow sewage exfiltration where the sewer is above a groundwater resource. Because of a lack of OK data on sewage exfiltration, research work in Germany is discussed and analogies are made between the sewerage networks of the two countries.Until recentll: it has not been possible to positively identify sewage contamination of groundwater because of other pollutants and potential sources within an urban environment. The development of sewagefingerprinting techniques has overcome these difficulties and provides conclusive evidence of sewage contamination of shallow and deep groundwater resources in researched areas. Studies of groundwater recharge and total solute loadings allow quantification of sewage exfiltration reaching the groundwater within the Midlands conurbation. The paper concludes that urban aquifers are potentially more vulnerable to microbiological contamination from leaking sewers than has previously been assumed.

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