Iowa Statewide Stream Nitrate Load Calculated Using In Situ Sensor Network

Abstract

Various techniques exist to estimate stream nitrate loads when measured concentration data are sparse. The inherent uncertainty associated with load estimation, however, makes tracking progress toward water quality goals more difficult. We used high‐frequency, in situ nitrate sensors strategically deployed across the agricultural state of Iowa to evaluate 2016 stream concentrations at 60 sites and loads at 35 sites. The generated data, collected at an average of 225 days per site, show daily average nitrate‐N yields ranging from 12 to 198 g/ha, with annual yields as high as 53 kg/ha from the intensely drained Des Moines Lobe. Thirteen of the sites that capture water from 82.5% of Iowa's area show statewide nitrate‐N loading in 2016 totaled 477 million kg, or 41% of the load delivered to the Mississippi–Atchafalaya River Basin (MARB). Considering the substantial private and public investment being made to reduce nitrate loading in many states within the MARB, networks of continuous, in situ measurement devices as described here can inform efforts to track year‐to‐year changes in nitrate load related to weather and conservation implementation. Nitrate and other data from the sensor network described in this study are made publicly available in real time through the Iowa Water Quality Information System.

Assessment of Spatial Nitrate Patterns in An Eastern Iowa Watershed Using Boat-Deployed Sensors

“…Similar studies have been performed on large freshwater lakes such as Lake Michigan [4]. Since that time, technological advancements have enabled in situ measurements of specific conductivity (SPC), nitrate-nitrogen (NO 3 -N), turbidity, chloride, pH, chlorophyll a and many other parameters from moving boats, floating buoys, and fixed monitoring stations anchored to the shoreline and bridge abutments [5][6][7][8][9]. Both fixed and boat-deployed stations Water 2020, 12, 146 2 of 13 produce temporally-dense water quality data, while data generated from a moving boat adds a high-density spatial dimension to in situ water quality research [6,[10][11][12].…”

“…The Iowa and Cedar Rivers combine at Columbus Junction, IA and the river then flows 47 km before discharging into the Upper Mississippi River near Toolesboro, IA. The ICRB is well understood to contribute a significant load of nitrate to the Gulf of Mexico and thus it provides a meaningful platform to study nitrate transport in a system known for high nitrate export [9,13]. Water 2020, 12, x FOR PEER REVIEW 3 of 13 waterways in the basin, with several large tributaries delivering water to each.…”

“…The Nitratax has features including automatic cleaning of optical surfaces and background subtraction for turbidity and dissolved organic carbon that enable measurement in turbid river water. More details about the device can be found at [9]. Temperature (T), specific conductivity (SPC), pH, and dissolved oxygen concentration (DO) were quantified with Hydrolab DS5X multiparameter sonde (OTT Hydromet).…”

“…The Loken et al [13] study showed disproportionately large NO 3 -N inputs from the intensely-farmed Iowa River Basin, which includes two major interior Iowa rivers, the Cedar and the Iowa. Other research has also shown the basin (hereafter designated as the Iowa-Cedar River Basin, ICRB) to be a strong contributor to Mississippi River NO 3 -N. Jones et al [9] found the NO 3 -N flux of the Iowa River at Wapello (which captures both Cedar and Iowa River flows) to be 14 kg ha −1 and 281 Mg in 2016. Upstream of the confluence, Jones et al [15] found the Cedar and Iowa River NO 3 -N yields to be 21.1 and 22.1 kg ha −1 , respectively, for the period 1999-2016.…”

“…The ICRB is a large area spanning multiple Iowa landform areas [17] and is 350 km north-south and 250 km east-west. Nitrate loss from these landforms varies based on land cover, slope, extent of artificial drainage, and weather [9,18]. Identifying areas of greatest NO 3 -N loss could help guide expenditure of conservation funds targeted toward improved water quality.…”