Changes in water chemistry and fluvial geomorphology from arsenic contaminated floodplains of Whitewood Creek and Belle Fourche River, South Dakota
Date
2023
Journal Title
Journal ISSN
Volume Title
Abstract
From 1877 to 1977 the Homestake Gold Mine in Lead, South Dakota released over 100 million megagrams (Mg) of arsenic rich mine waste into Whitewood Creek which joins the Belle Fourche River. The mine waste which contains arsenopyrite and other arsenic bearing minerals, is deposited along the floodplains of Whitewood Creek and the Belle Fourche River as overbank deposits and abandoned meander and channel fill. The introduction of mine tailings into these streams has impacted them chemically and geomorphologically for over 100 years. This study is a continuation of the work from Ji (2021) who focused on the long-term behavior of arsenic in the mine tailings. Her work involved sequential extractions of the tailings to determine the mineralogical setting of the arsenic and its rate of release. She also used statistical regression on historical data to estimate the physical and chemical removal of arsenic from Whitewood Creek's watershed. The focus of this study is to see how the tailings might have impacted the stream chemistry of Whitewood Creek and the Belle Fourche River by modeling mineral saturation indices of the stream and seep water through the geochemical modeling program, The Geochemist's Workbench. The Geochemist's Workbench was used to model the dissolution rate of arsenopyrite to calculate the rate of dissolved arsenic entering Whitewood Creek. Suspended arsenic entering Whitewood Creek was calculated using the dimensions of the creek bed, thickness of tailings, and density of arsenopyrite. In addition to chemistry, this study investigated the changes in the tailings and fluvial geomorphology of Whitewood Creek and the Belle Fourche River from 1948 to 2012. This was performed by using aerial photographs from 1971, which mapped locations of the tailings along the floodplains, and overlaying them with photographs from 1948, 1977, and 2012. Using GIS through ArcMap, the tailings and their portions that have been removed over time were digitized. Other fluvial parameters that have been determined and digitized are stream longitudinal profiles, sinuosity, contaminated floodplain width, channel migration, and total sediment deposition area. The mineral saturation indices of Whitewood Creek and the Belle Fourche River are similar to each other and differ at the most by around 2-3 orders of magnitude. The minerals that are supersaturated are mainly phyllosilicates (mostly clays), Fe, Cu and Al (hydr)oxides, and carbonates with minor sulfates and phosphates. Seep waters have lower mineral saturation indices, up to 10 orders of magnitude lower for Fe bearing minerals. The only arsenic bearing mineral that is calculated to be supersaturated is Ba3(AsO4)2; however, this mineral has not been observed in nature. Based on the range of possible arsenopyrite concentration in the contaminated sediment (15 to 0.11%), the calculation of dissolved arsenic being discharged out of Whitewood Creek ranges from 52 to 0.39 Mg per year. This range compares to Ji's (2021) daily dissolved arsenic rate range of 3.89-0.33 Mg/year. For a tailings width range of 0.6 to 3.5 m, the calculated rate of suspended arsenic being discharged ranges from 254 to 1.98 Mg per year. Although large, this range encompasses Ji's (2021) suspended arsenic transport rate range of 33 to 70 Mg per year. The overlap of values from Ji's (2021) statistical approach and this study's geochemical approach indicates that arsenopyrite may be to some degree significant in controlling As transportation in Whitewood Creek. Based on GIS results, the location and evolution of contaminated floodplains along Whitewood Creek and the Belle Fourche River are very complex. The streams are different from each other and behave as their own systems. In Whitewood Creek, locations with high tailings area and removal are controlled by a possible range of factors such as knickzone geomorphology, bedrock lithology, and changes in stream energy due to topography. In the Belle Fourche River, reaches with high tailings area and removal are found about 7 km from the Whitewood Creek confluence and a 30 km stretch where rapid floodplain reworking occurs due to neotectonics from Precambrian basement adjustments. Tailings removed area and contaminated floodplain width graphs show that the Belle Fourche River has larger storage for tailings and undergoes more floodplain reworking due to higher flood frequency and neotectonics. In contrast, Whitewood Creek has lower storage and erosion due to decreasing mine sediment load at least since 1948 and channel incision into shale bedrock in some reaches. While the reworking of tailings into the stream is lower in Whitewood Creek than the Belle Fourche River, the tailings will remain on the floodplains for many generations.
Description
Zip file contains Belle Fourche River and Whitewood Creek GIS data files.
Rights Access
Subject
Belle Fourche River
mine tailings
Whitewood Creek
fluvial geomorphology
arsenic geochemistry
South Dakota