Tile Drainage Density Reduces Groundwater Travel Times and Compromises Riparian Buffer Effectiveness

Abstract

Strategies to reduce nitrate-nitrogen (nitrate) pollution delivered to streams often seek to increase groundwater residence time to achieve measureable results, yet the effects of tile drainage on residence time have not been well documented. In this study, we used a geographic information system groundwater travel time model to quantify the effects of artificial subsurface drainage on groundwater travel times in the 7443-ha Bear Creek watershed in north-central Iowa. Our objectives were to evaluate how mean groundwater travel times changed with increasing drainage intensity and to assess how tile drainage density reduces groundwater contributions to riparian buffers. Results indicate that mean groundwater travel times are reduced with increasing degrees of tile drainage. Mean groundwater travel times decreased from 5.6 to 1.1 yr, with drainage densities ranging from 0.005 m (7.6 mi) to 0.04 m (62 mi), respectively. Model simulations indicate that mean travel times with tile drainage are more than 150 times faster than those that existed before settlement. With intensive drainage, less than 2% of the groundwater in the basin appears to flow through a perennial stream buffer, thereby reducing the effectiveness of this practice to reduce stream nitrate loads. Hence, strategies, such as reconnecting tile drainage to buffers, are promising because they increase groundwater residence times in tile-drained watersheds.

Right practice, right place: A conservation planning toolbox for meeting water quality goals in the Corn Belt

“…Trap and treat nutrient removal practices, which can intercept and treat both particulate and dissolved nutrients along flow paths from cropland to receiving surface waters, are critical to achieving large reductions in nutrient export. However, for these practices to be effective they must intercept the actual flow of water and nutrients; for example, riparian buffers located above tile drains may successfully trap sediment and particulate phosphorus (P) in surface runoff but provide little or no benefit for removing dissolved P and N from tile drainage (Schilling et al 2015a). Determining the most effective ("priority") practices for a given watershed therefore depends on understanding nutrient flow paths.…”

Groundwater monitoring at the watershed scale: An evaluation of recharge and nonpoint source pollutant loading in the Clear Creek Watershed, Iowa

“…An initial top-down approach was applied to the Clear Creek Watershed building from past studies (Elhakeem et al, 2017;Schilling, Streeter, et al, 2015;Schilling, Wolter, et al, 2015), considering surface-subsurface interactions. GRUs were developed for Clear Creek following previously developed hydrologic response units (Schilling, Streeter, et al, 2015;Schilling, Wolter, et al, 2015), which are a combination of slope, soil, and management, as well as pedotransfer functions, which link the surface and subsurface through infiltration (Elhakeem et al, 2017). In select GRUs, 13 monitoring wells were installed in Clear…”

“…Groundwater flow from distal watershed areas to Clear Creek could take many years or decades to occur, providing ample opportunities for nutrient attenuation along groundwater flow paths (Kornelsen & Coulibaly, 2014;Pint, Hunt, & Anderson, 2003;Robinson, 2015). However, in agricultural regions underlain by tile drainage systems, groundwater travel times are significantly reduced (Schilling, Streeter, et al, 2015;Schilling, Wolter, et al, 2015). In the SIDP landform region, tile drainage is primarily located in grassed waterways that occupy many first-order drainage ways.…”

“…In the SIDP landform region, tile drainage is primarily located in grassed waterways that occupy many first-order drainage ways. Schilling, Streeter, et al (2015) and Schilling, Wolter, et al (2015) estimated that this type of dendritic tiling decreased mean watershed travel times from 40 to 19 years in a Southern Iowa watershed. Determining the true groundwater travel time distribution in Clear Creek, and the time lag between nutrient sources and the stream, will require more detailed information on the complex distribution of groundwater flow paths in the watershed, including the use of groundwater flow models (Basu, Jindal, Schilling, Wolter, & Takle, 2012).…”

Agriculture and the Life Aquatic: Effects of Agricultural Landscape Structure on Farmland Aquatic Biodiversity and Water Quality

“…Drainage is implemented to remove excess water from fields for successful crop growth, and is achieved using surface and subsurface methods (Pavelis, 1987;Blann et al, 2009). Land drainage results in extensive wetland loss, physical modification of aquatic habitats, and increased rates of pollutant loadings on aquatic systems (Blann et al, 2009;Schilling et al, 2015). For example, subsurface drains significantly reduce groundwater travel time and the potential for runoff remediation via water infiltration through soil and riparian buffers (Schilling et al, 2015).…”