Grout lined storm drains are used worldwide to convey storm runoff away from urban areas. They also convey dry weather flows such as car wash runoff, lawn runoff, and other "nuisance flows" in urban centers. Areas in Southern California and elsewhere have shallow water tables where dry weather flows in grout lined storm drains consist primarily of perennial groundwater baseflows. In these areas, groundwater leaks through cracks, joints, weepholes, and intentional dewatering structures into storm drains and eventually flows into natural channels, coastal wetlands, and estuaries. The "lined" sections of these channels are not always impervious; in fact, the lining of the channels often affords excellent opportunities to collect groundwater samples at discrete points that are usually not present along unlined channels where well control is sparse or where technical or legal issues prevent installation of piezometers. Frequently, as much as 95% of the nutrient and trace element loading to urban catchments during dry weather is from seepage of groundwater that is laden with pollutants. This project presents several examples of our experiences in conducting stream/aquifer studies in lined and semi-lined channels in Southern California. Sub-projects include evolution of groundwater quality along flowpaths, detection of unwanted recharge mounds in shallow groundwater systems due to leaky water main pipes, and comparisons of water quality in lined and unlined sections of a creek. Tools that are utilized and described in the paper include isotopic tracers; trace element and standard inorganic constituents; and innovative sampling methods.

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Sulfate isotopes (δ34S, δ18OSO4) interpreted in conjunction with sulfate concentrations show that sulfate of both agricultural and geologic sources is present in groundwater and surface water in the Rio Grande flood plain within the Hueco Bolsón. From previous studies, water isotopes (δ2H, δ18O) in the study area indicate groundwater age relative to dam construction upstream. Surface water entering the Hueco Bolsón contains a mixture of soil-amendment sulfate and sulfate from deep-basin groundwater seeps at the terminus of Mesilla Valley. In the shallow Rio Grande alluvial aquifer within the Hueco Bolsón, ranges of δ34S in pre-dam (+2 to +9‰) and post-dam (0 to +6‰) groundwater overlap; the range for post-dam water coincides with common high-sulfate soil amendments used in the area. Most post-dam groundwater, including discharge into agricultural drains, has higher sulfate than pre-dam groundwater. In surface water downstream of Fabens, high-δ34S (>+10‰) sulfate, resembling Middle Permian gypsum, mixes with sulfate from upstream sources and agriculture. The high- δ34S sulfate probably represents discharge from the regional Hueco Bolsón aquifer. In surface water downstream of Fort Hancock, soil-amendment sulfate predominates, probably representing discharge from the Rio Grande alluvial aquifer near the basin terminus. The δ18OSO4 dataset is consistent with sulfate origins determined from the larger δ34S dataset.

Identification of recharge areas in arid basins is challenging due to spatial and temporal variability and complexity of the hydrogeology. This study re-evaluates recharge mechanism in a desert basin where isotopic and geologic data indicated that published conceptual models of recharge are not accurate. A new model of recharge is formulated that is consistent with the unique geologic framework in the basin. In the area of study, the Rio Grande flows across a broad alluvial floodplain, the “El Paso-Juarez Valley”, where the river has incised the surface of the Hueco Bolson. The modern Rio Grande floodplain overlies the older basin fill, or “Hueco Bolson deposits”, in the valley portion of the area. The lateral contact between the older bolson deposits and the recent alluvial floodplain deposits defines the “slope front”. The valley wall along the slope front is penetrated by many arroyos that incise the Hueco Bolson deposits and modern floodplain surface. The presence of a large lens of freshwater at the boundary between the older bolson fill and recent Rio Grande alluvium seemed to suggest to previous researchers that dilute water developed due to runoff drawn in by San Felipe Arroyo, a prominent arroyo at the slope front between the older Hueco Bolson deposits and the recent Rio Grande alluvium. Our follow-up verification work illustrates that this is demonstrably not the case. The testing of groundwater samples for stable water isotopes and radioisotopes showed that the deeper and more dilute waters near San Felipe Arroyo are actually pre-dam waters recharged from the shifting Rio Grande channel.

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