Measuring and Calculating Current Atmospheric Phosphorous and Nitrogen Loadings to Utah Lake Using Field Samples and Geostatistical Analysis
Total Page：16
File Type：pdf, Size：1020Kb
hydrology Article Measuring and Calculating Current Atmospheric Phosphorous and Nitrogen Loadings to Utah Lake Using Field Samples and Geostatistical Analysis Jacob M. Olsen, Gustavious P. Williams * ID , A. Woodruff Miller and LaVere Merritt Department of Civil and Environmental Engineering, Brigham Young University, Provo, UT 84602, USA; [email protected] (J.M.O.); [email protected] (A.W.M.); [email protected] (L.M.) * Correspondence: [email protected]; Tel.: +1-801-422-7810 Received: 20 July 2018; Accepted: 14 August 2018; Published: 15 August 2018 Abstract: Atmospheric nutrient loading through wet and dry deposition is one of the least understood, yet can be one of the most important, pathways of nutrient transport into lakes and reservoirs. Nutrients, specifically phosphorus and nitrogen, are essential for aquatic life but in excess can cause accelerated algae growth and eutrophication and can be a major factor that causes harmful algal blooms (HABs) that occur in lakes and reservoirs. Utah Lake is subject to eutrophication and HABs. It is susceptible to atmospheric deposition due to its large surface area to volume ratio, high phosphorous levels in local soils, and proximity to Great Basin dust sources. In this study we collected and analyzed eight months of atmospheric deposition data from five locations near Utah Lake. Our data showed that atmospheric deposition to Utah Lake over the 8-month period was between 8 to 350 Mg (metric tonne) of total phosphorus and 46 to 460 Mg of dissolved inorganic nitrogen. This large range is based on which samples were used in the estimate with the larger numbers including results from “contaminated samples”. These nutrient loading values are significant for Utah Lake in that it has been estimated that only about 17 Mg year−1 of phosphorus and about 200 Mg year−1 of nitrogen are needed to support a eutrophic level of algal growth. We found that atmospheric deposition is a major contributor to the eutrophic nutrient load of Utah Lake. Keywords: Utah Lake; phosphorus; nitrogen; atmospheric deposition; eutrophication; harmful algal blooms 1. Introduction 1.1. Study Goals and Importance Nutrients have been the focus of considerable scientific research in relation to eutrophication and also the growth of harmful algal blooms (HABs) [1,2]. Nutrients can enter an aquatic ecosystem in a variety of ways: surface tributaries, storm and wastewater discharges, groundwater, and from the atmosphere. Of these different avenues, atmospheric deposition is commonly neglected during the formation of a nutrient budget as in most areas it is not considered a significant source. Recently, with the increased focus on eutrophication of water bodies by nitrogen (N) and phosphorus (P), studies have found that atmospheric deposition is an important part of the total nutrient budget for many aquatic ecosystems [3–6]. Shallow lakes such as Utah Lake might be even more susceptible to atmospheric deposition because of their large surface area to volume ratios. Despite these characteristics, atmospheric deposition is commonly neglected with respect to the nutrient budgets of Utah Lake [7]. The objective of this work is to: (1) quantify the atmospheric loading of total phosphorous (TP) and dissolved Hydrology 2018, 5, 45; doi:10.3390/hydrology5030045 www.mdpi.com/journal/hydrology Hydrology 2018, 5, 45 2 of 20 inorganic nitrogen (DIN) to Utah Lake, (2) evaluate the spatial variability of TP and DIN loads over the surface of Utah Lake, and (3) characterize the temporal variability of TP and DIN loads among seasons. This case study is important as it clearly demonstrates, with field-sample data, the importance and magnitude of atmospheric nutrient loadings. For Utah Lake, and we suspect for many other lakes and reservoirs, reducing nutrient loads from other sources may be costly and because atmospheric loads were neglected, may not produce the expected results. In the case of Utah Lake, we show that atmospheric loads can support a eutrophic level of algae growth. 1.2. Utah Lake Water Quality Background There is increased interest recently in nutrient loadings to Utah Lake [8]. There is a controversy about the cost and effectiveness of reducing the large nutrient loadings from waste water treatment plants (WTTPs). There is agreement that the WTTPs contribute significant loads to Utah Lake, but there is disagreement on whether reducing or even eliminating these loads would significantly affect algal growth in Utah Lake. Currently phosphorous is not a limiting nutrient in Utah Lake and is present in excess. The debate is whether reducing WTTP TP loadings would be sufficient to reduce phosphorus loads such that they could be limiting to overall algal growth, as well as to limit HABs [7–9]. Many well-understood factors can be limiting for excessive algal growth and the occurrence of HABs. These include light levels, water clarity, water temperatures, slow-moving or stagnant waters, and high nutrient levels [10–12]. Of these factors, only nutrients can usually be controlled. In Utah Lake many of these factors affect algal growth and depending on climatology and seasonal effects can be limiting [13,14]. For example, at certain times of the year, light penetration (Secchi depth measurements of only a few centimeters) can be so low as to be a limiting factor in algal growth [7,8,15–17]. In 2004, Utah Lake was listed on Utah’s §303(d) list for exceedance of state criteria for total phosphorus (TP) and total dissolved solids (TDS) concentrations. The State of Utah’s Division of Water Quality conducted a study to calculate total phosphorus loadings to Utah Lake. The sources that were included in this study were tributaries (including WWTPs), groundwater, springs, and miscellaneous surface flows. Of the total phosphorus loading, the tributaries were found to contribute 97.2% [7]. Atmospheric deposition was not considered, mainly because little to no information was available. Since that time, Abu-Hmeidan, Williams and Miller [15] found that historic sediments in the lake have high phosphorous concentrations that affect water quality. Laboratory tests showed that Utah Lake sediments can contribute anywhere between 0.24 to 19 mg/L of TP to water in laboratory columns. This study also showed that soils on the Utah Lake shore are historic sediments that have high phosphorous values, up to 1000 mg/kg with average values over 700 mg/kg [15]. Merritt and Miller [9] completed an extensive eutrophic and nutrient loading evaluation for Utah Lake. They confirmed TP loadings from the earlier analysis by the Utah Department of Water Quality (DWQ) and studies by the engineering firm Psomas based on water inflows. They evaluated the impact of these loadings using the Larsen and Mercier Trophic State Model and the Carlson Trophic State Index Model [18,19]. They concluded that the actual lake state is currently moderately eutrophic, but P and N loadings estimated from water inflows to the lake are some 15 to 20 times larger than needed to support a eutrophic level of algal growth. They also found that for these estimated loadings, the N/P ratio was approximately 8, which indicates that nitrogen would be more limiting than P if nutrients are the limiting factor. They noted that this may have some impact, though tenuous, on the presence of HABs in the lake at times. Overall, Merritt and Miller [9] concluded that phosphorus and nitrogen loadings from water inflows could not be reduced sufficiently to make nutrients the limiting factors to algal growth. They reported that light limitation, via the high natural turbidity of the lake, was likely the overall limiting factor. They recommended quantification of atmospheric nutrient deposition to better understand the feasibility of nutrient control studies and make these studies more accurate. These recommendations were submitted before recent research that indicates that lake sediments may also be a large nutrient source to Utah Lake [15]. Based on the Larsen–Mercier Model, they concluded that only 17 Mg (tons) year−1 of phosphorus was needed to support the current moderately eutrophic Hydrology 2018, 5, 45 3 of 20 state of the lake (Note on units, English tons from previous studies have been directly converted to Metric tonnes (Mg) which is 1000 kg. This was done to maintain significant figures as the nutrient estimates have significant uncertainty and a more precise conversion would have resulting in numbers with higher precision than is warranted by the data). Using a N/P ratio of 12 for the algae, this means about 200 Mg (tons) year−1 of nitrogen would be needed by Utah Lake algae. Current estimated nutrient loadings are much higher than this, even without considering atmospheric deposition or sediment sources, and Merritt and Miller [9] noted it would not be feasible to reduce nutrient loadings to this level by improving WWTP effluent concentrations. 1.3. Deposition Clssification and Collection The National Atmospheric Deposition Program (NADP), the primary atmospheric deposition monitoring program in the United States, does not measure TP in precipitation samples. As a result, little data are available for atmospheric deposition of TP in Utah. However, there have been several studies performed in the Western United States with regard to nitrogen deposition. In these studies, nitrate deposition in the Utah Wasatch Front produced concentrations as high as 2.0 kg/ha N [20]. There are three main classifications of atmospheric deposition: bulk deposition, wet deposition, and dry deposition. Bulk deposition has been the most common form of atmospheric collection for decades [21,22]. Recently, it has become increasingly common to collect both wet and dry deposition. Wet deposition is defined as the process by which atmospheric pollutants become associated with cloud or precipitation droplets (or particles) and subsequently delivered to the earth’s surface during precipitation. Dry deposition is the process whereby gases and aerosols are deposited directly from the atmosphere to surfaces.