Agriculture has had direct and indirect effects on the rates and compositions of groundwater recharge and aquifer biogeochemistry Direct effects include dis- solution and transport of excess quantities of fertilizers and associated materials and hydrologic alterations relat- ed to irrigation and drainage Some indirect effects in- clude changes in water-rock reactions in soils and aqui- fers caused by increased concentrations of dissolved oxi- dants, protons, and major ions Agricultural activities have directly or indirectly affected the concentrations of a large number of inorganic chemicals in groundwater, for example NO 3 - , N 2 , Cl, SO 4 2- , H + , P, C, K, Mg, Ca, Sr, Ba, Ra, and As, as well as a wide variety of pesti- cides and other organic compounds For reactive con- taminants like NO 3 - , a combination of chemical, isoto- pic, and environmental-tracer analytical approaches might be required to resolve changing inputs from subse- quent alterations as causes of concentration gradients in groundwater Groundwater records derived from multi- component hydrostratigraphic data can be used to quan- tify recharge rates and residence times of water and dis- solved contaminants, document past variations in re- charging contaminant loads, and identify natural contam- inant-remediation processes These data indicate that many of the world's surficial aquifers contain transient records of changing agricultural contamination from the last half of the 20th century The transient agricultural groundwater signal has important implications for long- term trends and spatial heterogeneity in discharge

Groundwater recharge and agricultural contamination

The extensive development of surface and subsurface drainage systems to facilitate agricultural production throughout North America has significantly altered the hydrology of landscapes compared to historical conditions. Drainage has transformed nutrient and hydrologic dynamics, structure, function, quantity, and configuration of stream and wetland ecosystems. In many agricultural regions, more than 80% of some catchment basins may be drained by surface ditches and subsurface drain pipes (tiles). Natural channels have been straightened and deepened for surface drainage ditches with significant effects on channel morphology, instream habitats for aquatic organisms, floodplain and riparian connectivity, sediment dynamics, and nutrient cycling. The connection of formerly isolated wetland basins to extensive networks of surface drainage and the construction of main channel ditches through millions of acres of formerly low-lying marsh or wet prairie, where no defined channel may have previously existed, have r...

Effects of Agricultural Drainage on Aquatic Ecosystems: A Review

Phosphorus (P) loss from agricultural fields and watersheds has been an important water quality issue for decades because of the critical role P plays in eutrophication. Historically, most research has focused on P losses by surface runoff and erosion because subsurface P losses were often deemed to be negligible. Perceptions of subsurface P transport, however, have evolved, and considerable work has been conducted to better understand the magnitude and importance of subsurface P transport and to identify practices and treatments that decrease subsurface P loads to surface waters. The objectives of this paper were (i) to critically review research on P transport in subsurface drainage, (ii) to determine factors that control P losses, and (iii) to identify gaps in the current scientific understanding of the role of subsurface drainage in P transport. Factors that affect subsurface P transport are discussed within the framework of intensively drained agricultural settings. These factors include soil characteristics (e.g., preferential flow, P sorption capacity, and redox conditions), drainage design (e.g., tile spacing, tile depth, and the installation of surface inlets), prevailing conditions and management (e.g., soil-test P levels, tillage, cropping system, and the source, rate, placement, and timing of P application), and hydrologic and climatic variables (e.g., baseflow, event flow, and seasonal differences). Structural, treatment, and management approaches to mitigate subsurface P transport-such as practices that disconnect flow pathways between surface soils and tile drains, drainage water management, in-stream or end-of-tile treatments, and ditch design and management-are also discussed. The review concludes by identifying gaps in the current understanding of P transport in subsurface drains and suggesting areas where future research is needed.

Phosphorus transport in agricultural subsurface drainage: a review.

The midwestern United States offers some of the most productive agricultural soils in the world. Given the cool humid climate, much of the region would not be able to support agriculture without subsurface (tile) drainage because high water tables may damage crops and prevent machinery usage in fields at critical times. Although drainage is designed to remove excess soil water as quickly as possible, it can also rapidly transport agrochemicals, including phosphorus (P). This paper illustrates the potential importance of tile drainage for P transport throughout the midwestern United States. Surface runoff and tile drainage from fields in the St. Joseph River Watershed in northeastern Indiana have been monitored since 2008. Although the traditional concept of tile drainage has been that it slowly removes soil matrix flow, peak tile discharge occurred at the same time as peak surface runoff, which demonstrates a strong surface connection through macropore flow. On our research fields, 49% of soluble P and 48% of total P losses occurred via tile discharge. Edge-of-field soluble P and total P areal loads often exceeded watershed-scale areal loadings from the Maumee River, the primary source of nutrients to the western basin of Lake Erie, where algal blooms have been a pervasive problem for the last 10 yr. As farmers, researchers, and policymakers search for treatments to reduce P loading to surface waters, the present work demonstrates that treating only surface runoff may not be sufficient to reach the goal of 41% reduction in P loading for the Lake Erie Basin.

Surface runoff and tile drainage transport of phosphorus in the midwestern United States.

The phrase “hot spots and hot moments” first entered the lexicon in 2003, following the publication of the paper “Biogeochemical hot spots and hot moments at the interface of terrestrial and aquatic ecosystems” by McClain and others (Ecosystems 6:301–312, 2003). This paper described the potential for rare places and rare events to exert a disproportionate influence on the movement of elements at the scale of landscapes and ecosystems. Here, we examine how the cleverly named hot spot and hot moment concept (hereafter HSHM) has influenced biogeochemistry and ecosystem science over the last 13 years. We specifically examined the extent to which the HSHM concept has: (1) motivated research aimed at understanding how and why biogeochemical behavior varies across spatiotemporal scales; (2) improved our ability to detect HSHM phenomena; and (3) influenced our approaches to restoration and ecosystem management practices. We found that the HSHM concept has provided a highly fertile framework for a substantial volume of research on the spatial and temporal dynamics of nutrient cycling, and in doing so, has improved our understanding of when and where biogeochemical rates are maximized. Despite the high usage of the term, we found limited examples of rigorous statistical or modeling approaches that would allow ecosystem scientists to not only identify, but scale the aggregate impact of HSHM on ecosystem processes. We propose that the phrase “hot spots and hot moments” includes two implicit assumptions that may actually be limiting progress in applying the concept. First, by differentiating “hot spots” from “hot moments,” the phrase separates the spatial and temporal components of biogeochemical behavior. Instead, we argue that the temporal dynamics of a putative hot spot are a fundamental trait that should be used in their description. Second, the adjective “hot” implicitly suggests that a place or a time must be dichotomously classified as “hot or not.” We suggest instead that each landscape of interest contains a wide range of biogeochemical process rates that respond to critical drivers, and the gradations of this biogeochemical topography are of greater interest than the maximum peaks. For these reasons, we recommend replacing the HSHM terminology with the more nuanced term ecosystem control points. “Ecosystem control” suggests that the rate must be of sufficient magnitude or ubiquity to affect dynamics of the ecosystem, while “points” allows for descriptions that simultaneously incorporate both spatial and temporal dynamics. We further suggest that there are at least four distinct types of ecosystem control points whose influence arises through distinct hydrologic and biogeochemical mechanisms. Our goal is to provide the tools with which researchers can develop testable hypotheses regarding the spatiotemporal dynamics of biogeochemistry that will stimulate advances in more accurately identifying, modeling and scaling biogeochemical heterogeneity to better understand ecosystem processes.

https://par.nsf.gov/servlets/purl/10054793

Control Points in Ecosystems: Moving Beyond the Hot Spot Hot Moment Concept

While some of the world's most productive agriculture is on artificially drained soils, drainage is increasingly perceived as a major contributor to detrimental off‐site environmental impacts. Howe...

Hydrologic and water quality impacts of agricultural drainage

DRAINMOD-N: a nitrogen model for artificially drained soils

DRAINMOD-N, a quasi two-dimensional model that simulates the movement and fate of nitrogen in shallow
water table soils with artificial drainage, is described. Results of sensitivity analyses are presented and model predictions
are compared with results from VS2DNT, a more complex, two-dimensional model. The nitrogen transport component is
based on an explicit solution to the advective-dispersive-reactive (ADR) equation. Nitrate-nitrogen is the main N pool
considered. Functional relationships are used to quantify rainfall deposition, fertilizer dissolution, net mineralization,
denitrification, plant uptake, and surface runoff and subsurface drainage losses. 
A sensitivity analysis showed DRAINMOD-N predictions are most sensitive to the standard rate coefficients for
denitrification and mineralization and nitrogen content in rainfall. Simulated daily water table depths were within
0.121 m, cumulative subsurface drainage rates were within 0.016 m, and cumulative surface runoff rates were within
0.003 m, of those predicted by VS2DNT for a 250-day period. DRAINMOD-N predictions for NO3-N losses in subsurface
drainage water only differed from VS2DNT predictions by less than 2.6 kg ha–1. DRAINMOD-N predictions for
denitrification were within 8%, for plant uptake were within 15%, and for net mineralization were within 26%, of those
simulated by VS2DNT.

Drainmod-n, a nitrogen model for artificially drained soil

Using the DRAINMOD-N model to study effects of drainage system design and management on crop productivity, profitability and NO3-N losses in drainage water

This study was conducted to evaluate the performance of DRAINMOD-N, a nitrogen fate and transport model
for artificially drained soils, based on a comparison between predicted and observed hydrologic and nitrogen variables
for an experimental site in eastern North Carolina. The site consisted of six plots drained by subsurface drain tubes
1.25 m deep and 23 m apart. Each plot was instrumented to measure water table depth, subsurface drainage, surface
runoff and subirrigation rates. There were two replications of three water management treatments: conventional
drainage, controlled drainage and subirrigation. Crops were winter wheat followed by soybean. Results showed the
model did a good job in describing the hydrology of the site. On average the predicted daily water table depths were
within 0.13 m of observed during the 14-month study period. Differences between predicted and observed cumulative
subsurface drainage and surface runoff volumes were less than 0.10 and 0.09 m, respectively, for all treatments.
Predictions for the movement and fate of nitrogen were also in good agreement with measured results. Simulated nitratenitrogen
(NO3-N) losses in subsurface drainage water were within 1.5 kg/ha of the observed values for the 14-month
period. Differences between simulated and observed total NO3-N losses (subsurface drainage plus surface runoff) were
within 3.0 kg/ha. 
Results of this study indicated DRAINMOD-N could be used to simulate nitrogen losses in poorly drained soils with
artificial drainage. The model, however, needs to be tested for longer periods of time and under different climatic
conditions and soil types, before it can be recommended for general use.

M. A. Breve

Papers

Hydrologic and water quality impacts of agricultural drainage

DRAINMOD-N: a nitrogen model for artificially drained soils

Drainmod-n, a nitrogen model for artificially drained soil

Using the DRAINMOD-N model to study effects of drainage system design and management on crop productivity, profitability and NO3-N losses in drainage water

Field testing of drainmod-n