Mapping pollutant source to enhance water quality conservation in agricultural watersheds: Nonpoint no more?

Abstract

The term “nonpoint-source pollution” (NPSP) has been used to describe land-sourced, precipitation-driven contamination of surface waters by nutrients, sediment, and land-applied wastes and chemicals since enactment of the US Clean Water Act (CWA, including amendments) in the 1970s. Congress recognized the complexities of NPSP sources, timing, and pathways when writing the CWA and therefore deferred NPSP control to state-administered programs, funded through the US Environmental Protection Agency (USEPA), under Section 319 of the CWA. CitationBraden and Uchtmann (1985) discussed the policy decisions made, funding programs initiated, and roles of federal agencies involved, including the USDA, the US Army Corps of Engineers (USACE), and the USEPA. Renaming the Soil Conservation Service as the Natural Resources Conservation Service was coincident with an expanded role for USDA to help reduce agricultural pollutants in water. Given the importance of agriculture to society and the uncertainties involved in bringing about NPSP reductions, NPSP programs were designed flexibly to give agriculturalists time to gain experience and document success in abating NPSP. Research efforts to understand NPSP transport/ delivery processes and mitigation options using soil and water conservation practices began in earnest with passage of the CWA. CitationSweeten and Reddell (1978) reviewed the then-current knowledge of pollutant sources and early modeling efforts, which focused mostly on erosion and sediment transport. CitationWalter et al. (1979) discussed the principles behind conservation effectiveness in slowing rainfall runoff, as well as transport of soluble and adsorbed agricultural pollutants. The potential importance of lag effects (CitationBraden and Uchtmann 1985) and tradeoffs among pollutants (CitationWalter et al. 1979) were recognized in these early years. Research efforts have continued to the present through extensive field-based experiments and simulation modeling, with the Conservation Effects Assessment Project (CEAP) providing a focus for organizing these efforts within the USDA and among its Land Grant University (LGU) partners since the early 2000s (CitationMoriasi et al. 2020; CitationOsmond et al. 2012).

Landscape-scale variations in soils, terrain, hydrologic flow paths, and agricultural management practices were recognized as challenges to NPSP assessment and control by CitationDuda and Johnson (1985), but they did not see these complexities as a deterrent to taking action, saying:

We likely will never have enough information on such a dynamic problem or enough funding to prescribe just the right treatment for a watershed to achieve water quality goals—as in a “wasteload” allocation traditionally prepared for point-source discharges. [We ask—was this an early troll of the Total Maximum Daily Load concept?] Instead, a common sense approach must be taken to target implementation of BMPs to the primary pollution-source areas (or hot spots) and then use compliance with water quality standards or removal of use impairment as a measure of how well the pollution control effort fares. [Later they clarify that] the key to cost-effective water quality improvement is targeting to individual hot spots of agricultural pollution, not entire watersheds, not entire counties. … These hot spots can often be identified by ephemeral gullies or agricultural activities near ditches and streams.

CitationDuda and Johnson (1985) were calling for conservationists to improve their understanding of the landscape to become better at conservation planning toward water quality improvement. However, after nearly 40 years of subsequent research, planning, and implementation on the use of conservation practices to achieve NPSP control, we can reject this quote’s implied assumption that water quality issues can be addressed by treating “hotspots” that are easily identified based on visual observation and professional judgment. In particular, experience in the US Midwest shows that nutrient pollution is virtually ubiquitous across artificially drained agricultural watersheds (CitationTomer et al. 2008). This is why in-channel treatments (e.g., wetlands and two-stage ditches) are seeing greater emphasis in watershed conservation and research (CitationKalcic et al. 2018). Additionally, we know that (1) current expenditures are not resulting in water quality improvements when decadal trends are identified (CitationTomczyk et al. 2023), and (2) current policy may actually disincentivize treatment of hotspots (in favor of maximizing “acres treated”) (CitationStephenson et al. 2022). The problem appears quite thorny, but in our opinion, a part of the solution lies in developing approaches to clearly identifying opportunities for efficiently reducing nutrient and sediment losses actively occurring in each individual watershed. Today, there are technologies that can facilitate this through parsing water quality issues in individual watersheds at a spatial scale that directly supports conservation management decisions and by inventory and ranking of sites suited to specific conservation practices. The purpose of this article is to show that technologies to achieve this are available and, in some places, ready for use in testing and development.

We argue that advances such as high-resolution Light Detection and Ranging (LiDAR) topographic survey data, terrain analyses, remotely sensed crop cover data, modernized soil surveys, and computerized data access and analyses can be leveraged to map relative sources of agricultural nutrients and sediments at a scale that is meaningful in watershed- and farm-scale conservation efforts. More specifically, the Agricultural Conservation Planning Framework (ACPF) (CitationTomer et al. 2013, Citation2015, Citation2017) provides three types of layered information for identifying and ranking candidate locations for conservation treatment at the scale of practice implementation—the individual field. These data are already available across most of the Midwest. To address nitrogen (N) losses in this region, land use and by-field crop rotation data can be used to estimate the spatial distribution of fertilizer-N applications, by field and across watersheds (discussed later). Second, high-resolution (ranging from <1 to 3 m) topographic information can be used to map cumulative overland flow pathways by which water moves across the landscape to streams, rivers, and lakes. This level of resolution allows for slope steepness and upstream contributing areas to be mapped in detail across any landscape. Using contributing area and slope distributions, risks of erosion and sediment transport can be mapped at greater resolution than was possible before LiDAR-derived topographic data became available. This means that maps of upslope contributing areas can be weighted to consider estimated by-field nutrient loadings and pathway-specific sediment transport risk, rather than just land area. Third, locations suitable for installation of a range of interceptive conservation practices can be identified using conservation practice placement tools found in the ACPF toolbox, including practices that treat subsurface drainage and others that slow overland flow. With this information, the upstream contributing area to each suggested practice location can be weighted to rank each location based not just on land area but also the proportion of watershed nutrient load intercepted and/or (depending on planning objectives) risk of sediment loss. Overall, these data and geographic analyses can be used to consistently identify locations meeting threshold criteria (i.e., qualifying as a “hotspot”), where a given conservation practice could be installed to provide measurable benefits. The qualified sites can then be ranked according to the costs and benefits of conservation-practice implementation (CitationBravard et. al 2022). These techniques can provide ranking/prioritization among sites to help watershed coordinators engage landowners in achieving water quality improvement goals.

While this mapping technology is available now, it is not yet fully mature, nor available everywhere. We therefore encourage watershed-scale experimentation to test this approach to rank proposed sites for conservation implementation, as the databases required are developed by state. We hypothesize that the use of by-field crop rotation and high-resolution elevation datasets can be used to enhance the effectiveness of conservation efforts for NPSP control. We encourage testing of this hypothesis in a variety of agricultural landscapes, including naturally and artificially drained, and using a range of conservation practices, alone and in combination. We believe that well designed, paired-watershed experiments (CitationKing et al. 2008; CitationTomer 2018), combined with effective social engagement (CitationRanjan et al. 2020) and advanced spatial data and analysis approaches, can foster greater success in watershed improvement efforts.

Citation

James, D.E., Tomer, M.D., Porter, S.A., Cruse, R.M. and Zimmerman, E., 2024. Mapping pollutant source to enhance water quality conservation in agricultural watersheds: Nonpoint no more?. Journal of Soil and Water Conservation, 79(5), pp.84A-89A. https://doi.org/10.2489/jswc.2024.0802A