Transactions of the ASABE Vol. 57(6): 1687-1696 © 2014 American Society of Agricultural and Biological Engineers ISSN 2151-0032 DOI 10.13031/trans.57.10202 1687 TECHNICAL NOTE: APPLICABILITY OF KRIGING FOR ESTIMATING GROUNDWATER FLOW AND NUTRIENT LOADS SURROUNDING PINEVIEW RESERVOIR, UTAH T. N. Reuben, D. L. Sorensen ABSTRACT. The total maximum daily load (TMDL) study for Pineview Reservoir in Ogden Valley, Utah, called for reduc- tions in phosphorus loads from irrigated agriculture and on-site wastewater treatment systems, but historical groundwater flow estimates and associated phosphorus loads were highly variable and based on little data. The objective of the present study was to quantify the spatial variability of groundwater flow, groundwater nutrient concentrations, and nutrient transport toward Pineview Reservoir and improve the groundwater flow and nutrient loading estimates for the reservoir using this information. ArcGIS kriging techniques were employed in analyzing high-frequency groundwater flow and grab-sample nutrient concentration data. Results from the study showed large spatial variations in groundwater flows and nutrient loadings. Spatial variation in flows was attributed to variations in hydraulic gradients and saturated thickness, while nutrient loading variations were attributed to source variations and nutrient flushing to groundwater due to snow- melt and irrigation water. Both agricultural and domestic nonpoint sources appeared to influence the nutrient loadings. The median total dissolved phosphorus concentration (32 μg P L -1 ) for five of the nine wells monitored was more than three-fold lower than the median (104 μg P L -1 ) for all nine wells, signifying large spatial variations. Spatial variation in flow rate had confidence intervals ranging from 1,518 to 5,077 m 3 d -1 on 6 May 2011 to 447 to 1,814 m 3 d -1 on 27 June 2011. Knowledge of the spatial distribution of nutrient concentrations and loading would help decision makers in deter- mining specific management practices that would help abate groundwater nutrient loading and its subsequent impact on surface water bodies. Keywords. GIS, Groundwater, Kriging, Nitrogen, Phosphorus, Spatial variability. utrophication of reservoirs in the U.S. inter- mountain west is usually attributed to increasing phosphorus concentrations in the water column, which results in cyanobacteria and algae growth to unacceptable concentrations (Hein, 2006). Pineview Reser- voir’s TMDL study (USEPA, 2011) stipulated the need to reduce both nitrogen and phosphorus loadings by 15% in order to abate eutrophication (Utah DEQ, 2002). The reser- voir is located in Ogden Valley, approximately 11 km east of Ogden City in Weber County, Utah. It is an impoundment of the Ogden River and receives water from the North, Middle, and South Forks of the Ogden River, Geertsen Creek, Spring Creek, various other smaller tributaries, and groundwater. Pineview Reservoir’s maximum storage capacity is approx- imately 140 × 10 6 m 3 (WBWQMC, 1990; Winkelaar, 2010). Groundwater flow and subsequent nutrient loading esti- mates to Pineview Reservoir have been made before (Avery, 1994; Miner et al., 1990; Reuben et al., 2011; Utah DEQ, 2002). The Clean Lakes Study estimated that the annual groundwater contribution from the water table aqui- fer to Pineview Reservoir was approximately 25 × 10 6 m 3 (Miner et al., 1990). Miner et al. (1990) estimated the groundwater flows based on data from five monitoring wells studied during the 1988 irrigation season, but the method used to calculate the groundwater flow was not described. Avery (1994) simulated groundwater flows in Ogden Valley and estimated that the annual groundwater contribution from the water table aquifer to Pineview Res- ervoir was approximately 33 × 10 6 m 3 . Avery (1994) en- countered problems with sparse data, especially for the water table aquifer, and therefore recommended further studies involving more monitoring wells over a longer monitoring period. A study conducted for the Utah De- partment of Environmental Quality (Utah DEQ, 2002) used the flow estimated by the Clean Lakes Study to estimate N and P loading to the reservoir. The researchers also recom- mended that more studies be conducted to fill data gaps. Reuben et al. (2011) determined surface and groundwa- ter flow rates and nutrient (nitrogen, phosphorus, iron, and dissolved organic carbon) loadings as well as the fate of phosphorus in Pineview Reservoir. They reported that an inflow of 3.4 × 10 6 m 3 year -1 from the water table aquifer, approximately 2% of the total annual reservoir inflow, car- ried 22% of the NO 3 + NO 2 -N (nitrate-N) and 2.6% of the dissolved phosphorus annual loads to Pineview Reservoir. Submitted for review in April 2013 as manuscript number SW 10202; approved as a Technical Note for publication by the Soil & Water Division of ASABE in October 2014. The authors are Thomas N. Reuben, ASABE Member, Postdoctoral Associate, and Darwin L. Sorensen, Research Professor, Utah Water Research Laboratory, Utah State University, Logan, Utah. Corresponding author: Darwin Sorensen, 8200 Old Main Hill, Utah State University, Logan, UT 84322-8200; phone: 435-797-3207; e-mail: darwin. sorensen @usu.edu E