Runoff effect on eutrophic lake water quality and heavy metal distribution in recent littoral sediment

Abstract

Multivariate statistical methods (hierarchical clustering analysis: HCA, and principal component analysis: PCA) were used to study the influence of runoff and other diffuse pollution sources on lake water chemistry of Hough Park lake in Central Missouri. In addition, heavy metal concentrations in lake littoral sediment were evaluated for enrichment and probable ecological risk. The abundance of macronutrients in the lake water column followed the order: Ca > Mg > TIC > K > Na > S > NO₃ – N > Fe > NH₃ – N > TP. Heavy metal concentrations in the lake water column were below acute and chronic level ecological guidelines. TN:TP ratios (range: 4.1–6.8) revealed nitrogen limitation of algal and other photosynthetic plant growth. The HCA showed two major clusters of similarity between the sampling points suggesting different pollution levels for the clusters. PCA 1, 2 and 3 reflected the influence of natural biochemical processes, atmospheric deposition and runoff respectively on lake water chemistry. The abundance of heavy metals and the normalizing element (Li) in littoral sediment (<63 μm fraction) samples analyzed in decreasing order were: Mn > Zn > Cr > Ni > Li > Cu > Pb > Cd > Hg. The average concentration of Cr, Mn and Ni in littoral sediment fraction exceeded the respective lowest effects level (LEL) threshold limit. Metal bioavailability in sediment fraction was low since the most labile metal species contained between 0% and 11% of the total metal content. Using the risk assessment code (RAC) criteria, only Mn posed a medium risk to the lake system.

Introduction

Industrial wastes, agricultural wastewaters, runoffs and atmospheric deposition are major sources of contamination of many surface waters. Runoffs for example cause sedimentation problems (Karlsson et al., 2010) in receiving streams, rivers and lakes. They can also transport toxic pollutants into catchments (Göbel et al., 2007) with potential transfer to the food chain. Transport of toxic materials is often enhanced through adsorption processes on small grain size fractions of soil or sediment and particulate materials of plant or animal origin. Excessive input of nutrients from both natural weathering and human activities causes eutrophication (excessive growth of photosynthetic plants) with serious water quality impairment. Some of the implications of algal blooms in freshwater systems include reduction in dissolved oxygen levels (Neal et al., 2000) leading to fish kill, loss of aesthetic value of the ecosystem, increased cost of source water treatment for drinking water production (supposing the water body is pumped for drinking water production) and nutrient enrichment of immediate catchment or watershed depending on the local geology. In many freshwater systems, phosphorus is the limiting nutrient (Rekolainen et al., 2005) in eutrophication and effective control of nitrogen and phosphorus levels often leads to improved water quality.

In the State of Missouri, sedimentation, macronutrients, and metals are problems affecting some surface waters according to the 303d list (Missouri Department of Natural Resources, 2008). The United States Environmental Protection Agency (US EPA) reported that mercury and other metals top the list of major contaminants in lakes, ponds and reservoirs in the State (US EPA, 2002). About forty bodies of waters in Missouri were designated as impaired due to the amount of mercury levels found in fish fillet samples analyzed in the past (Missouri Department of Natural Resources, 2004). The mean level of mercury recorded for largemouth bass fillet samples was 0.36 mg kg⁻¹ (Missouri Department of Natural Resources, 2005). Hough Park is listed as impaired due to mercury from atmospheric deposition (Missouri Department of Natural Resources, 2008). Apart from mercury toxicity and risk, other heavy metals such as cadmium chromium, copper, nickel, lead and zinc at unacceptable concentrations can seriously affect biota in view of their persistence and bioaccumulation potential. Human uses of aquatic ecosystems can also be compromised by contaminated sediments through reductions in the abundance of food or sport fish species or due to the imposition of fish consumption advisories (Missouri Department of Natural Resources, 2005).

This study determined the macronutrients (nitrate-N, nitrite-N, ammonia-N, total nitrogen, and total phosphorus) and heavy metal (Cd, Cr, Cu, Mn, Pb, Ni, Zn and Hg) concentrations in a small eutrophic lake system listed as impaired by mercury and evaluated the effect of runoff to lake water chemistry through hierarchical clustering and principal component analysis of data set obtained during the spring period sampling program when precipitation event was pronounced. We also examined the fractionation of heavy metals in recent littoral sediment geochemical phases for potential toxicity and ecological risk. According to Kalff (2003), the shallow water (littoral) zone is mostly larger for majority of inland water bodies systems than the deep water (pelagic) zone (Kalff, 2003). The examination of potential risk in this lake system will provide management information towards achieving pollution reduction goals and also serve as a reference for future monitoring program.