Date of Award

3-22-2024

Document Type

Thesis

Degree Name

Master of Science (MS)

First Advisor

Eric W Peterson

Abstract

The Midwest states have highly fertile soils representing one of the world's most intense agricultural production areas. The agricultural lands in the Midwest, including Illinois, are drained using tiles to improve the soil conditions for high productivity. However, the discharge of the tile waters directly into streams short-circuits the soil’s role in nutrient cycling (plant uptake, denitrification, microbial immobilization) and contributes to elevated nitrate as nitrogen (NO3--N) concentrations in surface waters, causing algae blooms which impairs surface waters and degrade water quality. Most NO3--N exports from tile-drained watersheds in the Midwest occur from January to June, corresponding to periods of tile flow. One remediation strategy that has shown a reduction of nitrate concentrations from tile waters is saturated riparian buffer (SRB). Within the SRB, four soil lysimeters were installed at different locations near the area of the tile between wells 8 and 10 to collect pore water from the vadose zone. Three (3) sets of tensiometers were installed at 0.3 m and 0.6 m depth near the tile to record water tension. The main objective of this research was to examine and better define the influence of tile flow on the transport and fate of nitrate in the vadose zone of a saturated riparian buffer, answering two questions: 1) How does tile flow change the flow regime in the vadose zone? 2) Will nitrate be collected and transported from the vadose zone as the tile water saturates the zone. During periods of tile flow, the diversion box (water control structure) receives elevated levels of water that overflow into the lower chamber into the SRB. Flow into the SRB and rising of the water table saturates the soil with time as flow direction is downward; from a higher head at 30 cm to a lower head at 60 cm. During periods of no tile flow, the water table is lowered as water is not received into the SRB. Head begins to drop, and flow direction remains downward; from a higher head at well 8, 30 cm to a lower head at well 8, 60 cm with a greater gradient at 60 cm; suggesting that active plants are drawing water for growth hence creating greater tension at well 8, 60 cm. Reversal of gradient is seen at well 10. Statistical comparison between the means of two independent groups, nitrate as nitrogen concentrations (NO3- -N) within wells (8 D and 10 D) and NO3- -N within waters of the vadose zone at depths of 30 cm and 60 cm during periods of tile flow and no tile flow showed a statistical difference in well 10D as compared to 8D and waters of the vadose zone. Nitrate as nitrogen concentrations within the vadose waters and upgradient of the tile (well 8) were significantly different (lower) from the downgradient (well 10) during periods of tile flow. During periods of no tile flow, nitrate concentrations were similar throughout the groundwater. Results from the statistical comparison (t-test) showed the tile had an influence on the downgradient groundwater of the tile compared to the vadose zone waters and the upgradient groundwater of the diversion tile. This means that the tile introduced waters with higher nitrate as nitrogen concentrations that impacted the downgradient groundwater (well 10) compared to the vadose waters and upgradient groundwater (well 8). KEYWORDS: Nitrate; Saturated riparian buffer; Vadose zone; Upgradient of the diversion tile; Downgradient of the diversion tile; Lysimeter; Tensiometers

Comments

Imported from Akrofi_ilstu_0092N_12603.pdf

DOI

https://doi.org/10.30707/ETD2024.20240618063947586846.999998

Page Count

63

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