The Shallow and Deep hypothesis: Subsurface Vertical Chemical Contrasts Shape Nitrate Export Patterns from Different Land Uses
Zhi, W., & Li, L. (2020). The shallow and deep hypothesis: Subsurface vertical chemical contrasts shape nitrate export patterns from different land uses. Environmental Science & Technology, 54(19), 11915-11928. doi: 10.1021/acs.est.0c01340
- Eutrophication has threatened water resources worldwide, yet mechanistic understanding on controls of nutrient export remains elusive. This work tests the shallow and deep hypothesis: subsurface vertical chemical contrasts regulate nitrate export patterns under different land use conditions.
- We synthesized data from 228 watersheds and used reactive transport modeling (500 simulations) under broad land use, climate, and geology conditions.
- Data synthesis indicated that human perturbation has amplified chemical contrasts in shallow water (e.g., soil water) versus deep waters (e.g., groundwater), inducing primarily flushing patterns (concentrations increase with streamflow) in agriculture lands and dilution patterns (concentrations decrease with streamflow) in urban watersheds.
- Results revealed a quantitative relationship between export patterns and shallow-versus-deep concentration contrasts, underscoring the often-overlooked role of nutrient distribution over depth.
- Nitrate concentration under different land uses
- Nitrate concentration as a function of land uses
- Validation of the estimated shallow (Csw) and deep water (Cdw)
- Dependence of export pattern on subsurface concentration ratio
Higher nitrate concentrations generally cluster in the midwestern “Corn Belt” that is dominated by agricultural and mixed lands, whereas undeveloped and urban sites consistently have lower concentrations.
Statistical boxplot (Figure 3) confirmed that measured stream concentrations (median and mean in mg/L) decrease from agriculture (3.2, 4.0), to mixed (1.4, 2.5), to urban (0.58, 0.94), and to undeveloped (0.16, 0.26). The estimated deep water concentrations have a similar order, descending from agriculture (2.9, 3.0) to urban (1.2, 1.7) to undeveloped land (0.13, 0.25). Compared to undeveloped sites, agriculture and urbanization have elevated nitrate levels by 3−20 times.
Co-located measurements of stream, shallow water, and deep groundwater concentrations are rare, except in a few intensively measured sites.To validate the estimation of Csw and Cdw using stream concentration at the 95th and 5th percentiles, we compare the estimation from this work against literature data and data from the National GroundWater Monitoring Network (NGWMN) database.
The 228 sites used in this work have diverse land use, geology, and climate conditions. Despite the large variations in watershed conditions, the data converge toward the b curve prescribed by the equation. The Ag sites cluster around high b values at the top right corner, and urban sites scatter with a negative b value at the bottom left. The overwhelming convergence toward the S curve with high b values in agriculture lands indicates the first-order control of land use on concentration contrasts in shallow versus deep waters and export patterns. In other words, the vertical stratification in subsurface water chemistry, often driven by human activity, modulates the observed C-Q relations.
Nitrate data from a total of 228 sites across the contiguous United States were analyzed. This includes 147 sites from the National Water-Quality Assessment (NAWQA) project with, a national network established to monitor stream water quality since 1991. The NAWQA has daily discharge and monthly water quality data across four land use categories, i.e., agriculture, mixed, undeveloped, and urban. The NAWQA sites have median and mean drainage areas of 150 and 294 km2, respectively. The USGS Water Quality Watch (WQW) (https://waterwatch.usgs.gov/wqwatch/) has a total of 71 sites and has continuous real-time water quality data (5 minutes to hourly interval) since 2014. The WQW sites are primarily located in the Midwestern “Corn Belt” agricultural region (e.g., Iowa, Illinois, Indiana) with sizes ranging from 1 to > 29,000 km2. We also examined intensively-measured data in 6 undeveloped and 4 urban sites from research network including Critical Zone Observatories (CZOs, http://criticalzone.org/national/), Long Term Ecological Research Network (LTER, https://lternet.edu/), and Watershed Function Scientific Focus Area (SFA, https://watershed.lbl.gov/). The 6 undeveloped sites include Shale Hills watershed (CZO, Pennsylvania), East River and Coal Creek watersheds (SFA, Colorado), Sleepers River Research Watershed (Vermont), Hubbard Brook Experimental Forest W1 and W6 watershed (LTER, New Hampshire). The 4 urban sites are located within the Gwynns Fall Watershed (Baltimore Ecosystem Study LTER, Maryland), including Dead Run, Glyndon, Gwynnbrook, and Carroll Park. For USGS sites, discharge (or streamflow) and nitrate data were retrieved from the National Water Information System (https://waterdata.usgs.gov/nwis).
The sites were classified based on the percentage of major land use according to the following criteria from USGS. Agricultural sites have > 50% agricultural land and <= 5% urban land; urban sites have > 25% urban land and <= 25% agricultural land; undeveloped sites have <= 5% urban land and <= 25% agricultural land; all other combinations of urban, agricultural, and undeveloped land percent are classified as mixed land use. Land use in the NAWQA sites were from Spahr, et al. (2010). Land use in the WQW sites was compiled from multiple sources including EPA water quality reports, USGS watershed management plan, USDA watershed assessment report, and the National Land Cover Database. Out of the 228 sites, the number of agriculture, mixed, undeveloped, and urban sites are 61, 78, 49, and 40, respectively.
Supporting Information includes detailed site information, nitrate concentration, and C-Q slope b.