Monitoring groundwater-surface water interaction and nutrient mass exchange in the riparian corridor of the Lower Arkansas River Valley, Colorado
Date
2015
Authors
Huizenga, Alexander Paul, author
Bailey, Ryan, advisor
Gates, Timothy, advisor
Covino, Timothy, committee member
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Abstract
The Lower Arkansas River Valley in southeastern Colorado is an irrigated, agricultural valley suffering from high concentrations of nutrients (Nitrogen N; phosphorus P) and salts in the coupled groundwater-surface water system. The majority of data collection efforts and associated spatial analysis of concentrations and mass loadings from the aquifer to the stream network have been performed at the regional scale (> 500 km²). These regional scale assessments have indicated that river riparian areas play a major role in controlling nutrient mass flux to the Arkansas River and its tributaries. However, the water and nutrient mass exchange within the riparian-stream system have not yet been investigated in detail. The objective of this thesis is to enhance understanding of hydro-chemical stream-aquifer processes at the reach scale (< 5 km) along the main stem of the Arkansas River and along a major tributary. Using a suite of in-stream instruments and observation wells, a 4.7 km reach of the Arkansas River and a 2 km reach of Timpas Creek were monitored to quantify spatio-temporal groundwater-surface water interaction and mass inputs and outputs of nutrients. The total volume of water flowing into and out of each study reach was quantified using existing stream gages for upstream flow measurements and developing new stream gages for downstream flow measurements. Stage-discharge relationships were developed at the downstream locations using in-stream water level loggers and periodic flow measurements using Acoustic Doppler Velocimeters (ADVs). Monitoring included growing season length and 24-hour monitoring of flow and water quality. Using these monitoring data, mass balance calculations were used to quantify groundwater-surface water interactions and nutrient mass exchanges and loadings. For growing season length analysis, surface water samples were collected and in-situ measurements were made at the stream gaging sites every two weeks during the study period to provide a data set on fluxes into and out of each reach during the irrigation season. The two 24-hour sampling events were performed in June and October of 2014 to compare groundwater-surface water exchange and mass loadings at the beginning and end of the growing season. Composite water quality samples for total N, nitrate as nitrogen (NO₃‾; as N), nitrite as nitrogen (NO₂‾; as N), ammonium as nitrogen (NH₄⁺ as N), total P, and dissolved salts were collected at the gage locations every 2 hours using ISCO automatic samplers along with in-situ measurements of water level, temperature, and specific conductance. Water quality samples, along with in-situ measurements, were also collected from transects of shallow monitoring wells installed in the riparian corridor and on the banks of each reach during sampling events. These water quality data, as well as estimated gradients of groundwater hydraulic head between monitoring wells, were used to inform mass loading calculations. Growing season length monitoring results from the Arkansas River show decreases in NO₃‾ and total N concentrations ranging from 35% to 66% from upstream to downstream along the study reach. A growing season NO₃‾ mass balance performed on the Arkansas River indicated that 73% of the total NO₃‾ lost from the system can be attributed to in-channel and hyporheic processes. In addition, analysis of the water table elevations along the river suggest that there is an oscillation of the groundwater gradients during high flow periods. 24-hour monitoring suggests minimal upstream to downstream changes in total phosphorus loadings in the Arkansas River early in the growing season; however, there was a 29% increase in loadings in October. NO₃‾ loadings decreased 14% in June between the upstream and downstream monitoring stations, and an average of 41% in October. Groundwater and pore water sample results suggested extensive mixing of surface and groundwater in the Arkansas River, but indicated little exchange in Timpas Creek. These samples also suggest that denitrification occurs in both the riparian floodplain and hyporheic zones of the Arkansas River and Timpas Creek, while phosphorus immobilization and mobilization in groundwater is highly variable in these systems. These results provide a better understanding of hydro-chemical groundwater-surface interactions within the region and indicate the role of riparian and hyporheic zones in controlling and mitigating groundwater and surface water nutrient loadings to the stream network. The information derived from this study provides knowledge of hydro-chemical processes on small to medium spatial and temporal scales and provides a valuable contrast in controlling processes between main-stem and tributary riparian areas. This project also provides a database for future small to medium scale groundwater-surface water modeling efforts in the Lower Arkansas River Valley to further elucidate processes that govern nutrient mass transport in the riparian-stream system, with implications for regional-scale processes.
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nutrients
mass balance