**4. Research involving controlled subsurface irrigation and drainage with denitrification bioreactors at the David M. Barton Agriculture Research Center**

#### **4.1. Spring drainage water study: denitrification bioreactor inlet and outlet water chemistry for Spring 2015**

The 2015 growing season was the first operational year for the denitrification bioreactor. Nitrate-bearing tile drainage water from land cultivated to corn (*Zea mays* L.) entered the denitrification bioreactor during the "drainage season." Mean phosphate, ammonium, nitrate, and sulfate concentrations and water pH are presented (**Table 2**) to illustrate the baseline chemistry and document that tile drainage effluent has sufficient nitrate-N to be considered as an environmental hazard.


Tile drainage sampling (1A, 1B, 2A, 2B, 3B, and 4B), mean of 12 sampling times from 20 March 2015 to cessation of drainage on 6 July 2015.

**Table 2.** Mean phosphorus, ammonium, nitrate, and sulfate concentrations and pH of tile drainage waters collected during the spring 2015 drainage season.

Nitrate-N concentrations were substantially reduced by passage through the denitrification bioreactor, except for 29 May 2015 that was postnitrogen fertilization and a heavy rain event with large water volumes migrating through the bioreactor (**Figure 4**). From March through early May, the influx of nitrate-N averaged 17 mg NO3-N/L (standard deviation of 12 mg NO3- N/L), whereas the effluent concentrations were 5 mg NO3-N/L (standard deviation of 3 mg NO3N/L). Nitrate concentrations from late May to mid-June and following nitrogen fertilization, the influx of nitrate-N averaged 69 mg NO3-N/L (standard deviation of 31 mg NO3-N/L), whereas the effluent concentrations were 21 mg NO3-N/L (standard deviation of 40 mg NO3-N/L).

The associated corn biomass (**Figure 3**) demonstrates that nitrogen is primarily associated with grain (65%) and is thus removed from the soil landscape by harvest. Similarly, potassium (27% associated with grain) and phosphorus (74% associated with grain) demonstrate different

**4. Research involving controlled subsurface irrigation and drainage with denitrification bioreactors at the David M. Barton Agriculture Research**

**4.1. Spring drainage water study: denitrification bioreactor inlet and outlet water chemistry**

The 2015 growing season was the first operational year for the denitrification bioreactor. Nitrate-bearing tile drainage water from land cultivated to corn (*Zea mays* L.) entered the denitrification bioreactor during the "drainage season." Mean phosphate, ammonium, nitrate, and sulfate concentrations and water pH are presented (**Table 2**) to illustrate the baseline chemistry and document that tile drainage effluent has sufficient nitrate-N to be considered

**Sampling sites PO4-P NH4-N NO3-N SO4-S pH**

1A 0.3 0.9 21 2.7 6.8 1B 0.23 1.5 25.6 2.6 6.8 2A 0.19 1 16.4 2.7 6.6 2B 0.37 1 11.2 2 6.5 3B 0.2 0.7 15.8 1.6 6.9 4B 0.21 0.7 22 2.2 6.8 Bioreactor influx 0.23 0.8 59.1 4 6.7 Bioreactor effluent 0.25 0.9 38.6 2.1 6.6 Tile drainage sampling (1A, 1B, 2A, 2B, 3B, and 4B), mean of 12 sampling times from 20 March 2015 to cessation of

**Table 2.** Mean phosphorus, ammonium, nitrate, and sulfate concentrations and pH of tile drainage waters collected

Nitrate-N concentrations were substantially reduced by passage through the denitrification bioreactor, except for 29 May 2015 that was postnitrogen fertilization and a heavy rain event with large water volumes migrating through the bioreactor (**Figure 4**). From March through early May, the influx of nitrate-N averaged 17 mg NO3-N/L (standard deviation of 12 mg NO3- N/L), whereas the effluent concentrations were 5 mg NO3-N/L (standard deviation of 3 mg NO3-

**mg/L**

harvest removals.

12 Soil Contamination - Current Consequences and Further Solutions

**Center**

**for Spring 2015**

as an environmental hazard.

drainage on 6 July 2015.

during the spring 2015 drainage season.

**Figure 4.** Denitrification bioreactor nitrate-N concentrations at the receiving and exiting terminals.

Ammonium concentrations were not appreciably influenced by bioreactor passage. Ammonium-N concentrations were generally less than 1 mg NH4-N/L, except for 22 April 2015 (1.3 mg NH4-N/L influx and 0.4 mg NH4-N/L effluent) and 30 June 2015 (3.0 mg NH4-N/L influx and 2.4 mg NH4-N/L effluent). Phosphorus and sulfate concentrations and water pH were not appreciably influenced by fluctuations during the drainage season and were not significantly altered by denitrification bioreactor passage.

#### **4.2. Williams Creek impoundment and denitrification bioreactor efficiency**

In the winter of 2015, Williams Creek waters were pumped and impounded by a levee system and then allowed to infiltrate/percolate through the soil and entered the tile drainage system. Water captured by the controlled subsurface drainage technology was transported to the denitrification bioreactor.

#### *4.2.1. Williams Creek water and stop-log box 4B captured soil water*

Williams Creek water is classified as a calcium-carbonate type water with a pH range from 7.92 to 8.05, implying dissolved calcium carbonate was influencing pH. Soil water pH sampled from stop-log box 4B ranged from 6.36 to 7.15 with a mean near 6.75. Presumably, the soil's cation exchange complex buffered soil drainage water and reduced the pH of waters originating from Williams Creek.

The soil water comparisons for calcium, magnesium, potassium, and sodium (**Table 3**) reveal that calcium concentrations are greater in the Williams Creek impoundment trial than the spring 2015 drainage trial. The field was limed with calcite limestone in the winter of 2014– 2015 and limestone requires a lengthy time interval to dissolve, perform cation exchange, and complete acid neutralization, thus increasing the calcium saturation of the cation exchange complex. Additionally, Williams Creek may be assumed to be a water solute calcium source.


**Table 3.** Soil water concentrations of calcium, magnesium, potassium, and sodium.

Williams Creek waters show elevated nitrate concentrations, ranging from 12.7 mg NO3-N/L on 25 November 2015 to 672 mg NO3-N/L on 4 December 2015 (**Figure 5**). Soil water shows a nitrate-N increase to 33.1 mg NO3-N/L on 2 Dec 2015 and 44 mg NO3-N/L on 3 December 2015, suggesting that the soil resource is influenced by nitrate-N originating from Williams Creek. Soil water nitrate-N concentrations are consistently smaller than the water from Williams Creek, implying that the soil resource is reducing nitrate-N concentrations by a combination of two processes: (i) dilution of Williams Creek nitrate-N concentrations with the preexisting soil water and (ii) denitrification soil processes.

Nitrate-N concentrations in soil water after 7 December 2015 show a gradual decline. Between 27 November and 29 November 2015, approximately 2.94 in. of rainfall occurred, inferring that rainfall acted to dilute the soil water nitrate-N concentrations. Williams Creek and soil water both demonstrated greater nitrate concentrations on 2 December 2015.

*4.2.1. Williams Creek water and stop-log box 4B captured soil water*

14 Soil Contamination - Current Consequences and Further Solutions

ing from Williams Creek.

6/12/2015

12/13/2015

12/14/2015

Williams Creek water is classified as a calcium-carbonate type water with a pH range from 7.92 to 8.05, implying dissolved calcium carbonate was influencing pH. Soil water pH sampled from stop-log box 4B ranged from 6.36 to 7.15 with a mean near 6.75. Presumably, the soil's cation exchange complex buffered soil drainage water and reduced the pH of waters originat-

The soil water comparisons for calcium, magnesium, potassium, and sodium (**Table 3**) reveal that calcium concentrations are greater in the Williams Creek impoundment trial than the spring 2015 drainage trial. The field was limed with calcite limestone in the winter of 2014– 2015 and limestone requires a lengthy time interval to dissolve, perform cation exchange, and complete acid neutralization, thus increasing the calcium saturation of the cation exchange complex. Additionally, Williams Creek may be assumed to be a water solute calcium source.

**ID Ca (ppm) Mg (ppm) K (ppm) Na (ppm)**

4B 5.5 24 2.1 11.3 In 5.3 9.1 2.9 14.1 Out 8.9 11.6 2.8 13.8

4B 61 9.4 4.5 12.8 In 52 8.7 3.7 11.1 Out 54 8.6 3.5 11.2

4B 36 6 2.8 8.9 In 33.5 5.7 2.7 7.4 Out 35.5 5.8 2.6 8.3

Williams Creek waters show elevated nitrate concentrations, ranging from 12.7 mg NO3-N/L on 25 November 2015 to 672 mg NO3-N/L on 4 December 2015 (**Figure 5**). Soil water shows a nitrate-N increase to 33.1 mg NO3-N/L on 2 Dec 2015 and 44 mg NO3-N/L on 3 December 2015, suggesting that the soil resource is influenced by nitrate-N originating from Williams Creek. Soil water nitrate-N concentrations are consistently smaller than the water from Williams Creek, implying that the soil resource is reducing nitrate-N concentrations by a combination of two processes: (i) dilution of Williams Creek nitrate-N concentrations with the preexisting

Nitrate-N concentrations in soil water after 7 December 2015 show a gradual decline. Between 27 November and 29 November 2015, approximately 2.94 in. of rainfall occurred, inferring that rainfall acted to dilute the soil water nitrate-N concentrations. Williams Creek and soil water

**Table 3.** Soil water concentrations of calcium, magnesium, potassium, and sodium.

both demonstrated greater nitrate concentrations on 2 December 2015.

soil water and (ii) denitrification soil processes.

**Figure 5.** Nitrate concentrations from Williams Creek and stop-log box 4B. (Note: Log scale.) On 4 December 2015, Williams Creek showed 691 mg NO3-N/L. (Data not shown on graph for graphics clarity.) Pumping from Williams Creek stopped on 8 December 2015.

Ammonium concentrations are generally small, less than 2 mg NH4-N/L for Williams Creek and generally less than 1 mg NH4-N/L for soil waters. Williams Creek water has the greatest ammonium concentration on 7 December 2015 (1.7 mg NH4-N/L), approximately 3 days after the greatest nitrate-N concentrations, whereas soil water has the greatest ammonium concentration on 9 December 2015 (1.7 mg NH4-N/L). Mean phosphorus concentrations are 0.36 mg PO4-P/L for Williams Creek waters and 0.39 mg PO4-P/L for the field sampling site waters, with the concentration differences being not significant. These phosphorus concentrations are considered sufficiently abundant to support water eutrophication. Sulfate concentrations were not significantly different between the Williams Creek waters (mean SO4-S at 1.4 mg SO4-S/L) and the field sampling site waters (mean SO4-S at 1.2 mg SO4-S/L).

#### **4.3. Denitrification bioreactor nitrate reduction potential with Williams Creek source water**

pH of the denitrification bioreactor inlet and effluent waters were not significantly different for each sampling date; however, the inlet water pH varied from a low pH of 6.33 (30 November 2015) to pH 7.07 (12 December 2015) and the effluent water pH varied from pH 6.31 (30 November 2015) to pH 7.18 (12 December 2015).

Denitrification bioreactor outlet nitrate-N concentrations were slightly too appreciably smaller than the corresponding inlet nitrate-N concentrations (**Figure 6**). The highest nitrate-N concentrations occurred on 2 December 2015, which corresponds with the nitrate-N concentration rise associated with stop-log box 4B. Nitrate-N concentrations from 2 December to 7 December 2015 ranged from 35.1 mg NO3-N/L to 20.6 mg NO3-N/L for the inlet concentrations and from 25.3 mg NO3-N/L to 17.2 mg NO3-N/L for the outlet concentrations. From 8 December to 13 December 2015, the inlet and outlet nitrate-N concentrations became increasingly smaller, and the outlet nitrate-N concentrations continued to be smaller than those of the corresponding inlet concentrations.

**Figure 6.** Water nitrate concentrations from the inlet (influx) and outlet (effluent) from the denitrification bioreactor.

Ammonium-N concentrations were substantially smaller than the corresponding nitrate-N concentrations. Ammonium-N concentration differences between the inlet and outlet waters suggest that the denitrification bioreactor sequestered ammonium-N or nitrification processes oxidized ammonium to nitrate (**Figure 7**). Denitrification bioreactor's mean phosphorus concentrations were smaller for the effluent (0.29 mg PO4-P/L) than the inlet concentrations (0.38 mg PO4-P/L); however, the concentration differences were not significant. Denitrification bioreactor's mean sulfate concentrations were greater for the effluent (1.1 mg SO4-S/L) than the inlet concentrations (1.0 mg SO4-S/L); however, the sulfate-S concentration differences were not significant.

Denitrification bioreactors in these field trials reduced effluent nitrate-N concentrations via denitrification pathways. Approximately 50% or greater nitrate-N reductions were observed when the flow volumes per unit time were sufficiently small for equilibrium attainment.

**Figure 7.** Water ammonium concentrations from the inlet (influx) and outlet (effluent) from the denitrification bioreactor.
