Agricultural Tile Drainage Research Articles

Modeling the Effects of Tile Drain Placement on the Hydrologic Function of Farmed Prairie Wetlands

AbstractThe early 2000s saw large increases in agricultural tile drainage in the eastern Dakotas of North America. Agricultural practices that drain wetlands directly are sometimes limited by wetland protection programs. Little is known about the impacts of tile drainage beyond the delineated boundaries of wetlands in upland catchments that may be in agricultural production. A series of experiments were conducted using the well‐published model WETLANDSCAPE that revealed the potential for wetlands to have significantly shortened surface water inundation periods and lower mean depths when tile is placed in certain locations beyond the wetland boundary. Under the soil conditions found in agricultural areas of South Dakota in North America, wetland hydroperiod was found to be more sensitive to the depth that drain tile is installed relative to the bottom of the wetland basin than to distance‐based setbacks. Because tile drainage can change the hydrologic conditions of wetlands, even when deployed in upland catchments, tile drainage plans should be evaluated more closely for the potential impacts they might have on the ecological services that these wetlands currently provide. Future research should investigate further how drainage impacts are affected by climate variability and change.

Nitrogen Removal in Permeable Woodchip Filters Affected by Hydraulic Loading Rate and Woodchip Ratio.

Unregulated and event-driven agricultural tile drainage discharge poses several challenges that potentially limit the nitrate (NO) removal performance of woodchip-based wetlands constructed to intercept subsurface tile drain flows. Laboratory column tests were conducted to evaluate the biogeochemical response of mixed reactive media (woodchips-seashells and woodchips-Filtralite mixtures) at two woodchip ratios to changes in hydraulic loading rate (HLR). The tests involved continuous loading of aerated artificial drainage water spiked with NO-N and tritium (HO) breakthrough experiments. Flow-normalized NO reduction rates ranged from 0.35 to 3.97 g N m L, corresponding to N removal efficiencies of 5 to 64%, depending on HLR and filter mixtures. At high HLRs, oxic conditions prevailed in the woodchip filters, resulting in reduced N removal. At low HLRs, progressively lower pore-water velocities extended the period for consumption of terminal electron acceptors, increasing N removal. When increasing the content of mineral material, N removal declined, probably due to a lower denitrifying biomass at lower woodchip mass. The effect of woodchip ratios on solute transport characteristics was difficult to assess. However, woodchip media including a mineral fraction of crushed seashells demonstrated the highest N removal rates and efficiencies, most likely due to the alkalizing effect of the seashells. In conclusion, filter mixtures consisting of woodchips and seashells were the most effective material for N removal in subsurface flow-constructed wetlands treating agricultural drainage water.

Agricultural Drainage Filters. I. Filter Hydro-Physical Properties and Tracer Transport

Drainage filters using porous granular material constitute new innovative technologies for remediating phosphorus (P) from agricultural tile drainage water. In drainage filters where convective velocities are often high, we hypothesize that intragranular diffusion may affect solute transport depending on filter characteristics and flow rate. This was investigated for six drainage filter materials (Leca, Filtralite-P®, granulated limestone, crushed seashells, calcined diatomite earth (CDE), and a poorly ordered Fe oxide aggregate (CFH)) conducting a tritium (3H2O) tracer experiment at low (0.26 cm h−1), medium (23 cm h−1), and high (41 cm h−1) flux densities. The filter materials differed widely with respect to grain-size distribution (D 50 from 1.6 to 3.3 mm), uniformity coefficient (1.7 to 2.2), particle density (1.75 to 2.76 g cm−3), bulk density (0.34 to 1.46 g cm−3), and water-filled porosity (0.39 to 0.73 cm3 cm−3). Measurements of specific surface area (SSA) included both SSABET and SSAEGME to ensure inclusion of the intragranular microporosity, not accounted by N2-BET. SSA varied widely across methods and allowed the differentiation of filters according to the significance of the intragranular porosity. Tritium transport varied from approximately equilibrium transport at all flow rates in Leca, Filtralite-P®, and limestone, to progressive non-equilibrium transport as flow rate increased in Seashells, CDE, and CFH. In general, the filter materials were highly variable in hydro-physical properties. Filters with (approximately) equilibrium transport were, however, all characterized by low specific surface areas. The non-equilibrium transport was explained by an intragranular diffusion in filters with larger specific surface area (Seashells, CDE, and CFH).

Agricultural Drainage Filters. II. Phosphorus Retention and Release at Different Flow Rates

Granular drainage filters may provide new innovative solutions for removing phosphorus (P) from agricultural tile drainage water. The influence of filter sorbent properties is well recognized; however, intragranular diffusion may be critical for the P retention efficiency in drainage filters, where convective velocities are generally high. This was investigated in granular materials (Leca, Filtralite-P®, granulated Limestone, crushed Seashells, calcined diatomite earth (CDE), and poorly ordered Fe hydroxide aggregates (CFH)) with known solute transport behavior at variable flow rates. Phosphorus retention and release were investigated during continuous P loading with a constant inlet concentration of 0.3 mg P L−1 followed by a subsequent P release phase. A parallel batch sorption experiment ranked the materials according to strength of P retention CFH > Filtralite-P® > Seashells > Limestone > Leca, while CDE was a P source at low solution concentrations. Column experiments demonstrated that filters with non-equilibrium transport (CFH and Seashells) attained higher P retention rates and lower P release rates compared to filters with equilibrium flow (Filtralite-P® and Limestone). This behavior was even more pronounced as flow rate increased and was attributed to intragranular diffusion of P in CFH and Seashells, which would maintain concentration gradients between mobile convective water and immobile domains. This facilitated further diffusion and thus increased P retention. In addition, the release of previously retained P was limited in filters with non-equilibrium transport ( 5 %). The results demonstrated that intragranular diffusion was an important mechanism increasing P retention.

Performance of Agricultural Residue Media in Laboratory Denitrifying Bioreactors at Low Temperatures

Denitrifying bioreactors can be effective for removing nitrate from agricultural tile drainage; however, questions about cold springtime performance persist. The objective of this study was to improve the nitrate removal rate (NRR) of denitrifying bioreactors at warm and cold temperatures using agriculturally derived media rather than wood chips (WC). Corn ( L.) cobs (CC), corn stover (CS), barley ( L.) straw (BS), WC, and CC followed by a compartment of WC (CC+WC) were tested in laboratory columns for 5 mo at a 12-h hydraulic residence time in separate experiments at 15.5 and 1.5°C. Nitrate-N removal rates ranged from 35 to 1.4 at 15.5°C and from 7.4 to 1.6 g N m d at 1.5°C, respectively; NRRs were ranked CC > CC+WC > BS = CS > WC and CC ≥ CC+WC = CS ≥ BS > WC for 15.5 and 1.5°C, respectively. Although NRRs for CC were increased relative to WC, CC released greater amounts of carbon. Greater abundance of nitrous oxide (NO) reductase gene () was supported by crop residues than WC at 15.5°C, and CS and BS supported greater abundance than WC at 1.5°C. Production of NO relative to nitrate removal (NO) was consistently greater at 1.5°C (7.5% of nitrate removed) than at 15.5°C (1.9%). The NO was lowest in CC (1.1%) and CC-WC (0.9%) and greatest in WC (9.7%). Using a compartment of agricultural residue media in series before wood chips has the potential to improve denitrifying bioreactor nitrate removal rates, but field-scale verification is needed.

Controls Influencing the Treatment of Excess Agricultural Nitrate with Denitrifying Bioreactors.

Denitrifying bioreactors have been suggested as effective best management practices to reduce nitrate and nitrite (NO) in large-scale agricultural tile drainage. This study combines experiments in flow-through laboratory reactors with in situ continuous monitoring and experiments in a pair of field reactors to determine the effectiveness of reactors for small-scale agriculture in New York. It also compares the use of a typical woodchip media with a woodchip and biochar mixture. Laboratory results showed linear increase in NO removal with both increased inflow concentration and increased residence time. Average removal of NO in weekly monitoring of field reactors over the course of two growing seasons was 3.23 and 4.00 g N m d for woodchip and woodchip/biochar reactors, respectively. Removal of NO during two field experimental runs was similar to in situ monitoring and did not correlate with laboratory experiments. Factors that are uncontrollable at the field scale, such as temperature and inflow water chemistry, may result in more complex and resilient microbial communities that are less specialized for denitrification. Further study of other controlling variables, other field sites, and other parameters, including microbial communities and trace gas emissions, will help elucidate function and applicability of denitrifying bioreactors.

Phosphorus Retention in a Newly Constructed Wetland Receiving Agricultural Tile Drainage Water

One measure used in Sweden to mitigate eutrophication of waters is the construction of small wetlands (free water surface wetland for phosphorus retention [P wetlands]) to trap particulate phosphorus (PP) transported in ditches and streams. This study evaluated P retention dynamics in a newly constructed P wetland serving a 26-ha agricultural catchment with clay soil. Flow-proportional composite water samples were collected at the wetland inlet and outlet over 2 yr (2010-2011) and analyzed for total P (TP), dissolved P (DP), particulate P (PP), and total suspended solids (TSS). Both winters had unusually long periods of snow accumulation, and additional time-proportional water samples were frequently collected during snowmelt. Inflow TP and DP concentrations varied greatly (0.02-1.09 mg L) during the sampling period. During snowmelt in 2010, there was a daily oscillation in P concentration and water flow in line with air temperature variations. Outflow P concentrations were generally lower than inflow concentrations, with net P losses observed only in August and December 2010. On an annual basis, the wetland acted as a net P sink, with mean specific retention of 69 kg TP, 17 kg DP, and 30 t TSS ha yr, corresponding to a reduction in losses of 0.22 kg TP ha yr from the agricultural catchment. Relative retention was high (36% TP, 9% DP, and 36% TSS), indicating that small constructed wetlands (0.3% of catchment area) can substantially reduce P loads from agricultural clay soils with moderately undulating topography.

Vegetation Changes and Partitioning of Selenium in 4-Year-Old Constructed Wetlands Treating Agricultural Drainage

The knowledge of selenium (Se) partitioning in treatment wetlands and wetland vegetation management are essential for long-term effective operation of constructed wetlands treating Se-laden agricultural tile-drainage in central California. In this field study, samples from different compartments of treatment wetlands were collected and the vegetation change in each wetland cell was examined four years after the wetland's inception. The results showed that saltgrass (Distichlis spicata) and rabbitfoot grass (Polypogon monspeliensis) were less competitive than cattail (Typha latifolia) and saltmarsh bulrush (Scirpus robustus). Over 90% of the wetland cell originally vegetated with saltgrass or rabbitfoot grass was occupied by invasive plants—i.e., when invasive species were not controlled in the wetlands. More Se was likely found in sediments from vegetated regions, compared to the unvegetated areas of the wetland cell. Particularly, rhizosphere sediments accumulated about 4-fold more Se than non-rhizosphere sediments. Among the total Se retained in the wetland, 90% of the total Se was partitioned in the top 10-cm layer of sediment. The Se accumulation in plant materials accounted for about 2% of the total Se mass retained in each wetland cell. This field study demonstrated that wetland plants play significant roles in the treatment of Se-laden agricultural drainage.

Rates of Nitrate and Perchlorate Removal in a 5‐Year‐Old Wood Particle Reactor Treating Agricultural Drainage

AbstractA 5‐year‐old wood particle reactor treating agricultural tile drainage in southern Ontario was monitored for its ongoing ability to treat both nitrate (NO3–) and perchlorate (ClO4–). Prior to sampling undertaken in the fifth year of operation, a highway safety flare containing ClO4– was immersed in the inlet pipe elevating influent ClO4– concentrations to up to 33.7 μg/L. ClO4– removal rates were inhibited in the presence of more than 1 to 2 mg/L NO3–‐N, but increased rapidly to about 60 μg/L/d upon NO3– depletion. Nitrate removal rates, measured subsequently in the sixth and seventh years of operation, varied with temperature in the range of 2 to 16 mg N/L/d, but remained similar to rates measured in the second year. Additionally, no deterioration in the hydraulic conductivity (K) of the coarse core layer (0.5 <K < 5 cm/s) was detected over the monitoring period. These results demonstrate that coarse wood particle media can deliver stable NO3– removal rates and can remain highly permeable over a number of years. The media can also provide high removal rates for other redox sensitive contaminants such as ClO4–. The ability to directly measure the reactor flow rate, in this case via an outlet pipe, greatly simplified the task of estimating hydraulic properties and reaction rates.

Pharmaceutical and personal care products in tile drainage following surface spreading and injection of dewatered municipal biosolids to an agricultural field

Land application of municipal biosolids can be a source of environmental contamination by pharmaceutical and personal care products (PPCPs). This study examined PPCP concentrations/temporally discrete mass loads in agricultural tile drainage systems where two applications of biosolids had previously taken place. The field plots received liquid municipal biosolids (LMB) in the fall of 2005 at an application rate of ∼ 93,500 L ha − 1 , and a second land application was conducted using dewatered municipal biosolids (DMB) applied at a rate of ∼ 8 Mt dw ha − 1 in the summer of 2006. The DMB land application treatments consisted of direct injection (DI) of the DMB beneath the soil surface at a nominal depth of ∼ 0.11 m, and surface spreading (SS) plus subsequent tillage incorporation of DMB in the topsoil (∼ 0.10 m depth). The PPCPs examined included eight pharmaceuticals (acetaminophen, fluoxetine, ibuprofen, gemfibrozil, naproxen, carbamazepine, atenolol, sulfamethoxazole), the nicotine metabolite cotinine, and two antibacterial personal care products triclosan and triclocarban. Residues of naproxen, cotinine, atenolol and triclosan originating from the fall 2005 LMB application were detected in tile water nearly nine months after application (triclocarban was not measured in 2005). There were no significant differences ( p > 0.05) in PPCP mass loads among the two DMB land application treatments (i.e., SS vs. DI); although, average PPCP mass loads late in the study season (> 100 days after application) were consistently higher for the DI treatment relative to the SS treatment. While the concentration of triclosan (∼ 14,000 ng g − 1 dw) in DMB was about twice that of triclocarban (∼ 8000 ng g − 1 dw), the average tile water concentrations for triclosan were much higher (43 ± 5 ng L − 1 ) than they were for triclocarban (0.73 ± 0.14 ng L − 1 ). Triclosan concentrations (maximum observed in 2006 ∼ 235 ng L − 1 ) in tile water resulting from land applications may warrant attention from a toxicological perspective.

Phosphorus Losses through Agricultural Tile Drainage in Nova Scotia, Canada

Tile drainage water from agricultural fields commonly exceeds environmental guidelines for phosphorus (P) in rivers and streams. The loss of P through artificial drainage is spatially and temporally variable, and is related to local factors. This study characterizes variability in total P (TP) and soluble reactive P (SRP) concentrations in weekly drainage samples from 39 agricultural fields in Nova Scotia, Canada, from April 2002 through December 2003. We examined connections between P concentrations and the factors: (i) soil texture; (ii) discharge flow rate; (iii) soil test P (STP); (iv) manure type; and (v) crop cover. Generally, variability between fields and samples was great, and fields with standard deviations exceeding the mean for TP, SRP, and flow rate were 71, 54, and 79%, respectively. It was evident that poultry and swine manure contributed to high STPs, and to constantly high TP concentrations with high proportions of SRP. Concentrations varied from week to week, and particularly in April, May, October, and November when the greatest TP, SRP, and flow rate averages were measured. Mean TP concentrations exceed the USEPA (1994) TP guideline of 0.10 mg L(-1) at 82% of the fields, and periodically concentrations more than 10 times, and occasionally more than 50 times higher than the guideline were found. The proportion of SRP in TP had a tendency to be higher when TP levels were high in coarse textured soils. In Nova Scotia, dairy manure is most often applied on permanent cover crops, which did not show as much P concentration variability as crop rotations. Daily or hourly observation of short-term increases in P concentrations related to the described factors would help to characterize the changes in P concentrations observed during frequent heavy drainage flow events.

Dissolved Phosphorus Losses in Tile Drainage under Subirrigation

Abstract Recent studies have shown subirrigation (SI) to be effective in reducing nitrate losses from agricultural tile drainage systems. A field study was conducted from 2001 to 2002 in southwestern Québec to evaluate the effect of SI on total dissolved phosphorus (TDP) losses in tile drainage. In an agricultural field with drains installed at a 1-m depth, a SI system with a design water table depth (WTD) of 0.6 m below the soil surface was compared with conventional free drainage (FD). Subirrigation increased drainage outflow volumes in the autumn, when drains were opened and water table control was interrupted for the winter in the SI plots. Outflows were otherwise similar for both treatments. Throughout the study, the TDP concentrations in tile drainage were significantly higher with SI than with FD for seven out of 17 of the sampling dates for which data could be analyzed statistically, and they were never found to be lower for plots under SI than for plots under FD. Of the seven dates for which the increase was significant, six fell in the period during which water table control was not implemented (27 September 2001 to 24 June 2002). Hence, it appears that SI tended to increase TDP concentrations compared with FD, and that it also had a residual effect between growing seasons. Almost one-third of all samples from the plots under SI exceeded Québec's surface water quality standard (0.03 mg TDP L-1), whereas concentrations in plots under FD were all below the standard. Possible causes of the increase in TDP concentrations in tile drainage with SI are high TDP concentrations found in the well water used for SI and a higher P solubility caused by the shallow water table.

DENITRIFICATION OF AGRICULTURAL DRAINAGE USING WOOD-BASED REACTORS

Two denitrification reactor designs, utilizing alternate layers of fine and coarse wood particles, were monitored for their ability to achieve passive, maintenance-free nitrate removal in agricultural tile drainage. A lateral flow design was tested over a 26-month period on drainage from a cornfield in southern Ontario, and an upflow design was tested over a 20-month period on drainage from a golf course, also in southern Ontario. At the cornfield site, flow through the reactor averaged 7.7 L/min at an average influent NO3 concentration of 11.8 mg N/L, and removal averaged 3.9 mg N/L. At the golf course site, flow through the reactor averaged 7.8 L/min at an average influent NO3 concentration of 3.2 mg N/L, and removal averaged 1.7 mg N/L. Areal removal rates averaged 2.5 g N/m2/d in the cornfield reactor and 0.95 g N/m2/d in the golf course reactor, and are about an order of magnitude higher than rates reported for other passive treatment systems such as constructed wetlands even though average operating temperatures were relatively low (7°C to 9°C). Mass balance calculations indicate that carbon consumption from denitrification was <2% per year; thus, these reactors have the potential to operate for a number of years without the need for media replenishment. Both reactors were successful in achieving maintenance-free operation during all seasonal conditions, including unassisted startup after drought and freeze periods. Reactors such as these have the potential for a range of applications in agricultural settings because of their low cost and low maintenance characteristics. They are most usefully applied in the treatment of base flows rather than peak flows and can be readily used in combination with other treatment systems such as constructed wetlands.

Calcium sulfate (gypsum) scaling in nanofiltration of agricultural drainage water

Reclamation and reuse of agricultural tile drainage water in the San Joaquin Valley, California, by membrane desalination has been given serious consideration for nearly 30 years. The agricultural drainage water has very high levels of calcium and sulfate ions, rendering it nearly saturated with gypsum in some locations in this region. The present paper focuses on the conditions of membrane surface gypsum scale formation during nanofiltration (NF) of agricultural drainage water. Experimental studies with drainage water samples and model solutions were conducted in laboratory glassware as well as with a small plate-and-frame cross-flow membrane re-circulation unit. NF membrane experiments consisted of two types: permeate disposal to measure the effect of increasing recovery on the onset of scaling, and feed water re-circulation at a fixed concentration factor to establish performance change over time. An expression relating NF product water recovery to feed water concentration factor enabled numerous membrane scale formation studies that have direct relevance to full-scale NF systems. Glassware experiments showed that magnesium ions decreased the amount of gypsum incipient nuclei because of MgSO 4 0 complexation, which reduces the availability of sulfate ions for nucleus formation. In membrane experiments, model solutions showed that bicarbonate, magnesium ions, and humic acid retarded the onset of gypsum scaling by tying up calcium ions that would have otherwise been used for formation of gypsum incipient nuclei. Both calcium carbonate and gypsum scales may result from particulate deposition rather than membrane surface (wall) crystallization, with the likelihood of particulate fouling increasing with supersaturation.

Effectiveness of Constructed Wetlands in Reducing Nitrogen and Phosphorus Export from Agricultural Tile Drainage

AbstractMuch of the nonpoint N and P entering surface waters of the Midwest is from agriculture. We determined if constructed wetlands could be used to reduce nonpoint N and P exports from agricultural tile drainage systems to surface waters. Three treatment wetlands (0.3 to 0.8 ha in surface area, 1200 to 5400 m3 in volume) that intercepted subsurface tile drainage water were constructed in 1994 on Colo soils (fine‐silty, mixed, superactive, mesic Cumulic Endoaquoll) between upland maize (Zea mays L.) and soybean [Glycine max (L.) Merr.] cropland and the adjacent Embarras River. Water (tile flow, precipitation, evapotranspiration, outlet flow, and seepage) and nutrient (N and P) budgets were determined from 1 Oct. 1994 through 30 Sept. 1997 for each wetland. Wetlands received 4639 kg total N during the 3‐yr period (96% as NO3‐N) and removed 1697 kg N, or 37% of inputs. Wetlands decreased NO3−N concentrations in inlet water (annual outlet volume weighted average concentrations of 4.6 to 14.5 mg N L−1) by 28% compared with the outlets. When the wetlands were coupled with the 15.3‐m buffer strip between the wetlands and the river, an additional 9% of the tile NO3−N was apparently removed, increasing the N removal efficiency to 46%. Overall, total P removal was only 2% during the 3‐yr period, with highly variable results in each wetland and year. Treatment wetlands can be an effective tool in reducing agricultural N loading to surface water and for attaining drinking water standards in the Midwest.

The role of seepage in constructed wetlands receiving agricultural tile drainage

Constructed wetlands positioned in the landscape between row crop agriculture and surface waters can be used to intercept tile drainage and serve as agricultural waste water detention basins. A potential exit pathway in constructed wetlands for detained water and possibly NO 3 −-N is via seepage through and under an earthen berm. The objective of this study was to determine if seepage was an important pathway for NO 3 −-N transport from two constructed wetlands receiving tile drainage from adjacent agricultural land (wetland A, surface area of 0.6 ha; wetland D, 0.78 ha). A mean apparent hydraulic conductivity ( K) was calculated (10.8 cm h −1, range 8.2–14.3 cm h −1) using empirical water budgets. Using Darcy’s law, which included the apparent hydraulic conductivity, effective seepage area and daily hydraulic gradient measurements, daily seepage volumes for both wetlands were calculated for the 1997 water year. Total seepage volumes for wetlands A and D were 26 and 22 million liters, respectively, for the 1997 water year, which represented 47 and 27% of the total inlet flow. The amount of NO 3 −-N exiting wetlands A and D in seepage water was estimated to be 61 and 25 kg N, respectively, and represented 10 and 4% of the total inlet NO 3 −-N load. Seepage connected the wetland with the riparian buffer strip and transported NO 3 −-N to populations of denitrifiers deeper in the sediment profile and outside the wetland perimeter, thereby enhancing overall NO 3 −-N removal efficiencies.

An assessment of the potential hazards of environmental selenium for Canadian water birds

Elevated dietary intake of selenium (Se) causes dramatic teratogenic and other reproductive effects in water birds at dietary levels only a few times greater than those that are generally regarded as nutritionally essential. Possible risks to the health of Canadian water birds and other wildlife from Se may be posed by seleniferous habitats in western Canadian provinces, and over-wintering by migratory water birds in some National Wildlife Refuges in the western U.S.A. where Se-related deaths and embryonic deformities in birds have occurred. These Refuges have in common an arid climate, closed drainage basins, seleniferous Cretaceous geology, and they receive irrigation drainage water from nearby agricultural areas. Similar conditions occur in SW Saskatchewan and SE Alberta, and these areas overlap parts of the critical water bird habitat of the region. Selenium concentrations in wetland ponds, agricultural tile drainage water and bird tissues in the western provinces are, with a few exceptions, generally lower than threshold concentrations for reproductive toxicity. However, some surface waters and aquifers contain dissolved Se concentrations that exceed guidelines for the protection of aquatic life, irrigation uses, and human and livestock drinking water. Use of this aquifer water for irrigation or replacement of freshwater flows into wetlands would greatly increase the risk of Se bioaccumulation to hazardous levels in water bird food chains.Key words: selenium, toxicity, birds, wildlife, Canada.

In Situ Measurements of Denitrification in Constructed Wetlands

AbstractQuantitative estimates of denitrification are needed in designing artificial wetlands to optimize nitrate (NO−3) removal. Acetylene blockage and 15N‐tracer methods were employed to quantify denitrification in constructed wetlands receiving agricultural tile drainage, using plastic tubes to enclose in situ mesocosms. Estimates were also made through NO−3 disappearance from mesocosm water columns. The 15N and C2H2 methods yielded comparable rates. At 4 to 25°C, and with 9 to 20 mg NO−3‐N L−1 initially in the mesocosm water columns, denitrification rates by the C2H2 technique ranged from 2.0 to 11.8 mg N m−2 h−1. In the June–August 15N experiment, when wetland NO−3 was below detection, a time series of denitrification rates followed a bell‐shaped curve after a pulse input of NO−3 (∼15 mg N L−1, 70 atom% 15N). The maximal denitrification rate (9.3 mg N m−1 h−1) was observed 5.4 d after the pulse. After 33 d, 58% of the 15NO−3 had been evolved as N2, only ∼0.1% as N2O; 6 to 10% was recovered in plant shoots and as organic N in the upper 5 cm of sediment. From 32 to 36% of the 15NO−3 spike was not recovered, and presumably seeped into the sediments. The NO3 disappearance rates in the water column ranged from 12 to 63 mg N m−2 h−1 at 11 to 27°C. Because water infiltration carries NO−3 through the anaerobic sediment/water interface for denitrification, a subsurface‐flow wetland may denitrify more NO−3 than a surface‐flow wetland.

A simple model for predicting solute concentration in agricultural tile lines shortly after application

Abstract. Agricultural tile drainage lines have been implicated as a source of pesticide contamination of surface waters. Field experiments were conducted and a simple model was developed to examine preferential transport of applied chemicals to agricultural tile lines. The conceptual model consists of two linear reservoirs, one near the soil surface and one near the tile drain. The connection between the two reservoirs is via preferential flow paths with very little interaction with the soil matrix. The model assumes that only part of the field contributes solutes to the tile drain. The model was evaluated with data from the field experiments in which chloride, 2,4-D, and atrazine concentrations were measured on eight tile-drained plots that were irrigated twice. Atrazine was applied two months prior to the experiment, 2,4-D was sprayed just before the first irrigation, and chloride before the second irrigation. All three chemicals were found in the tile effluent shortly after the rainfall began. Generally, the concentration increased with increased flow rates and decreased exponentially after the rainfall ceased. Although the simple model could simulate the observed chloride concentration patterns in the tile outflow for six of the eight plots, strict validation was not possible because of the difficulty with independent measurement of the data needed for a preferential flow model applied to field conditions. The results show that, to simulate pesticide concentration in tile lines, methods that can measure field averaged preferential flow characteristics need to be developed.

Impact of atrazine-bearing agricultural tile drainage discharge on planktonic drift of a natural stream

Preliminary results are reported from a co-operative study between Agriculture Canada and Environment Canada on environmental impacts of atrazine measured in a field stream receiving agricultural drainage. Systematically, tile drained plots of known crop rotation, area, flow and pesticides use were used in the study. A maximum tile drainage concentration of 13.9 μg liter −1 atrazine was measured while the maximum measured stream concentration was 1.89 μg liter −1. Phytoplankton and zooplankton samples were collected on a bi-monthly basis during the growing season. The study indicated possible negative impacts of low concentrations of atrazine on planktonic drift populations, when natural stream flow was reduced, resulting in a lower dilution capacity. A 20 m section of the stream was affected by the tile drainage waters as measured by the resident biological community. Both atrazine and ambient environmental conditions were felt to be contributing to the measured results. No negative impacts on planktonic drift populations were evident beyond 50 m downstream of the tile drainage and stream confluence.