The land use legacy effect: looking back to see a path forward to improve management

Abstract

Water quality has suffered as humans have increased nutrient inputs across the landscape. In many cases, management actions to reduce nutrient inputs have not been met with concomitant ecosystem responses. These missed expectations are partly due to the continued slow delivery of nutrient-enriched groundwater pre-dating input reductions resulting from management actions. Land use legacies as expressed through this time lag are important to quantify in order to adjust management expectations. We present a novel coupling of nitrogen source maps with groundwater transport times to create a high-resolution (120 m) fully distributed estimate of the timing and magnitude of groundwater nitrogen deliveries to surface water across Michigan’s Lower Peninsula. This new view of the landscape has been designed around common management timelines for: elected officials looking to make a difference for re-election (<5 years), career managers hoping to see the fruits of their labor (5–30 years), and advocacy groups whose work can span generations (>30 years). One striking result is that after 100 years, in our study area, approximately 50% of the nitrogen that enters the groundwater system remains in transit. This means that actions taken now may not show the expected lower nitrogen loads to receiving ecosystems for decades to centuries. We show that differences in groundwater travel times create a heterogeneous patchwork over which managers can prioritize actions to best match their targeted response times. Across the highest nitrogen inputs in our study region, less than 10% had short enough groundwater legacies to match the management timeline of most government and agency work. Agricultural practices (manure and chemical fertilizer) are the main nitrogen contributors across the top three management classes; however, human contributions through septic tank effluent and lawn fertilizers contribute 5%–8% of nitrogen.

Limited progress in nutrient pollution in the U.S. caused by spatially persistent nutrient sources

“…Environmental legislation such as the U.S. Clean Water Act has been largely successful in reducing point source pollutants, including wastewater effluent, feedlot outflows, and industrial discharge [12]. These decreases in nutrient inputs are having measurable effects on some water quality parameters, which should be recognized and celebrated [24,79,80]. However, our study and other recent work [14,23,81] highlight that the goal of the Clean Water Act "to restore the chemical, physical, and biological integrity of U.S. waters" has not been fully realized.…”

“…This downward trajectory is certainly encouraging, but it largely represents an incremental step toward the range of the earliest surveys' nutrient conditions, and could be due to interannual hydrological variability or non-stationarity more generally associated with other factors. One explanation for this water quality plateau could be that we will not see major improvements until nutrient legacies are depleted [16,80]. If legacies are the cause of the lack of progress, current nutrient management efforts should be sustained where in place and applied to areas that currently are not managed.…”

“…This emphasizes the need to craft regional solutions and ensure entities or individuals at the local level are PLOS ONE empowered to achieve them (i.e., subsidiarity). The needed information and resources to achieve further improvements include: 1) continued innovation in nutrient use efficiency in agriculture [55], 2) educational outreach and incentives to further improve nutrient management on farms [108,109], 3) documenting nutrient inputs and surpluses across the landscape to identify inefficiencies in the handling and use of N and P [50], 4) identifying areas of the landscape disproportionately degrading downstream water quality [80,98], and 5) establishing clear watershed-wide nutrient goals towards which local communities can strive [81,110]. Certainly, to assess the effectiveness of local decisions in achieving water quality goals, standardized tracking of management actions (e.g., fertilizer use, cover crops) and continued water quality monitoring is essential.…”

Examining Relationships Between Groundwater Nitrate Concentrations in Drinking Water and Landscape Characteristics to Understand Health Risks

“…Non‐point sources include atmospheric deposition, chemical agricultural fertilizer, manure, chemical non‐agricultural fertilizer, septic tanks, and nitrogen fixation from legumes. SENSEflux (Martin et al., 2021) is an expanded and updated model based on previous work by Luscz et al. (2017) that implements a statistical transport model to quantify the amount of nitrogen that survives transport to the Great Lakes coastline after attenuation along surface and groundwater pathways.…”

“…Variable groundwater travel times create a time lag or legacy, complicating the risk of contaminated drinking water as nitrate from surface inputs may take decades to propagate through the system (Fenton et al., 2017; Kim et al., 2020; Martin et al., 2017; Van Meter & Basu, 2015; Vero et al., 2018). It is critical to identify populations at risk and adapt current management practices to mitigate future threats to health (Ascott et al., 2021; Hansen et al., 2017; Martin et al., 2021).…”