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Prospects for enhanced groundwater recharge via infiltration of urban storm water runoff: a case study.

Article Excerpt
Urban landscapes are a mosaic of pervious and impervious areas, the latter of which reduces the extent of infiltrative surface, leading to elevated excess storm water runoff during storm events. One approach to the mitigation of urban storm water runoff is to install best management practices (BMPs) at smaller spatial scales (i.e., the parcel scale) and thereby decentralize management. This approach can serve to disconnect impervious areas from storm sewers and prevent the formation of runoff (Walsh 2004). A popular storm water BMP is the rain garden, which incorporates a capacity for surface storage and infiltration of storm water into an attractive horiticultural setting (Holman-Dodds et al. 2003; Morzaria et al. 2004; Hunt et al. 2005). The runoff that might otherwise end up in storm sewers or as sheet flow across landscapes is thereby routed to the rain garden where it is infiltrated and redistributed throughout the rooting zone and surrounding native soils or transpired. The use of rain gardens as storm water BMPs generally centers on the reduction of excess storm water runoff, improving or protecting water quality, and accomplishes these objectives by managing the most frequent rainfall events that comprise the majority of annual precipitation totals.

An important outcome of decentralized storm water management via infiltration BMPs may be to increase the amount of water available for groundwater recharge. Depending on subsurface flow paths and the architecture of underlying geological strata, groundwater recharge may flow down gradient to supply base flow in streams (Gobel et al. 2004), which can have a positive impact on aquatic habitat and biota (Brandes et al. 2006). Water resources directed to subsurface flow pathways may be alternatively lost to cracked or rotted geological formations, intercepted by roots and transpired, or conveyed along the faces of impermeable substrata to eventually provide stream base flow.

Given the explicit dependence of rain garden performance on soil conditions, the spatial distribution of soils and their comparative hydrology can be an important factor (Jenkinson et al. 2002; Tugel et al. 2005) in understanding the prospects for rain gardens to perform acceptably and generate recharge. In particular, the hydraulic properties of the subsoil surrounding a rain garden are an important regulator of both rain garden performance and the ecosystem services (i.e., enhancement of base flow) that it can provide. The relative timing of the rain garden drydown from a saturating event and the next rain event will regulate infiltration capacity and the potential for overflow from the BMP. The amount of water potentially available for deep percolation and recharge would therefore greatly rely upon a thorough characterization of hydrology of representative pedons (Barner 1999) and their distribution as soil map units in a watershed. The use of infiltrometers at appropriate depth in the subsoil would be expected to improve on qualitative estimates of subsoil hydraulic conductivity (USDA 1992) that are customarily made in order 2 soil surveys (Blanco-Canqui et al. 2002). For these reasons, we hypothesize that differences between estimated and measured subsoil hydraulic conductivity will affect simulated rain garden performance and its potential contribution to local and watershed recharge. The objectives of this study were to apply detailed soil maps, qualitative and measured subsoil saturated hydraulic conductivity ([K.sub.sat]) data, and natural rainfall records to a model of rain garden hydrology so as to estimate the potential for recharge on the basis of parcel-scale rain gardens and their hypothetical arrangement within a small urban watershed.

This work is conducted in the context of current research into a legal, socially acceptable method for management of storm water quantity through the installation of parcel-scale BMP retrofits (figure 1; Roy et al. 2005) via an incentive program. The selected retrofit BMPs are rain gardens and rain barrels to disconnect and otherwise minimize impervious area that is directly connected to a stream channel. The incentive program (Parikh et al. 2005) is an auction that accepts bids from landowners for the BMPs. The bid reflects landowner values regarding decentralized storm water management, opportunity costs of dedicating privately owned land to storm water management objectives, and other nonmarket values. Bids were collected, ranked in ascending order, and then weighted on the basis of objective criteria that would affect effectiveness (e.g., area of directly connected impervious area, soil runoff potential, and proximity to a stream reach). The BMPs are subsequently installed at no charge to the landowners for which bids were found acceptable. The bid amount is paid out to the landowner, which is the "willingness to accept" cost to the BMP authority for the landowner to accept a BMP on their property. We presently study the implementation of this storm water management approach and its possible benefits in a small watershed near Cincinnati, Ohio. An important part of this research...

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