Quantification of heavy metals and metalloids in soil samples collected from grazing sites and streams around Cove, AZ

The focus of this work is to investigate potential uranium, vanadium, arsenic, and manganese contamination in soils because of previous mining activities in Cove. Topsoil samples from grazing sites were collected across Cove to assess its potential impact on the plants of these sites. Soil cores were collected near streambeds to assess the deposition of metals and metalloids on the soil by stream water. Following the demand for uranium after World War II, approximately 30 million tons of uranium ore was mined on the Navajo Nation until mining ceased in 1986. Cove, a region and community located in northeastern Arizona between the Lukachukai and Carrizo mountains, was one of the productive areas for extracting uranium ore. When the mining operations stopped, the mines were abandoned and left open without remediation. The ore and remaining mine waste were left exposed to the surrounding environment, causing concern for elevated levels of heavy metals and metalloids in the soil, plants, and water. Contaminated water running through Cove may introduce these contaminants to the soils and plants. Plants growing in contaminated soils may take up the excess metals and metalloids within them. Residents of Cove have livestock that graze on the plants on their land, so these contaminated plants can introduce metals and metalloids to their livestock, which in turn exposes the residents to the heavy metals and metalloids when they eat the animals. Since it begins with the soils, quantification of their metal and metalloid contents is important to identify where livestock may be at higher exposure. Sample collection occurred on September 18-19, 2020, in a collaboration between Northern Arizona University and Diné College. All samples were processed, acid digested following EPA method 3051A, and analyzed for trace metal composition by either inductively coupled plasma – mass spectrometry (Thermo Fisher XSeries 2) or flame atomic absorption spectrometry (PerkinElmer AAnalyst 200 AA Spectrometer). The key results were the concentrations of uranium, vanadium, arsenic, and manganese were well below residential soil screening limits set by the New Mexico Environment Department. The soils from the streams generally had a higher content of metals and metalloids than the soils from the grazing sites. The soils from the grazing sites generally had low concentrations, and, in general, the stream soils collected from one foot in depth had a higher metal and metalloid content than the soil from the surface