Modeled Sources, Transport, and Accumulation of Dissolved Solids in Water Resources of the Southwestern United States1

Abstract

Information on important source areas for dissolved solids in streams of the southwestern United States, the relative share of deliveries of dissolved solids to streams from natural and human sources, and the potential for salt accumulation in soil or groundwater was developed using a SPAtially Referenced Regressions On Watershed attributes model. Predicted area-normalized reach-catchment delivery rates of dissolved solids to streams ranged from <10 (kg/year)/km2 for catchments with little or no natural or human-related solute sources in them to 563,000 (kg/year)/km2 for catchments that were almost entirely cultivated land. For the region as a whole, geologic units contributed 44% of the dissolved-solids deliveries to streams and the remaining 56% of the deliveries came from the release of solutes through irrigation of cultivated and pasture lands, which comprise only 2.5% of the land area. Dissolved-solids accumulation is manifested as precipitated salts in the soil or underlying sediments, and (or) dissolved salts in soil-pore or sediment-pore water, or groundwater, and therefore represents a potential for aquifer contamination. Accumulation rates were <10,000 (kg/year)/km2 for many hydrologic accounting units (large river basins), but were more than 40,000 (kg/year)/km2 for the Middle Gila, Lower Gila-Agua Fria, Lower Gila, Lower Bear, Great Salt Lake accounting units, and 247,000 (kg/year)/km2 for the Salton Sea accounting unit.

Freshwater salinization syndrome on a continental scale

“…For example, decreasing trends in total dissolved solids have been attributable to complex water diversions (mostly for agriculture) and decreased irrigation return flows of salts to river systems ( 35 ). Efficiency of agricultural and irrigation practices has also increased over time, which could also decrease salt accumulation rates in some drainages ( 44 , 45 ).…”

Dissolved-solids sources, loads, yields, and concentrations in streams of the conterminous United States

“…Comparison of results from several studies indicates that the general relative order of increasing dissolved-solids yields for rocks is crystalline (plutonic or metamorphic) rocks, volcanic rocks, and sedimentary rocks (Anning, 2011). Of the sedimentary rocks, sands, sandstones, and quartzite are generally the least susceptible to weathering, and they can have smaller relative yields than crystalline rocks.…”

“…Smith and others (1997) developed the SPARROW modeling technique and applied it to build an understanding of nitrogen and phosphorus sources and transport in streams of the conterminous United States. Subsequent efforts that have developed SPARROW models include but are not limited to nitrogen and phosphorus sources and transport for streams in specific major river basins of the United States (Preston, Alexander, and Wolock, 2011;Preston, Alexander, Schwarz, and Crawford, 2011), dissolved-solids sources and transport in streams of the Upper Colorado River Basin (Kenney and others, 2009), and dissolved-solids sources and transport in streams of the southwestern United States (Anning and others, 2007;Anning, 2011). SPARROW models have been used to (1) extrapolate known water-quality conditions in monitored reaches to estimate conditions in unmonitored reaches; (2) establish links between water quality and constituent sources; (3) track the transport of constituents to streams and downstream receiving waters, such as estuaries; (4) assess the natural processes that attenuate constituents as they are transported from land and downstream; and (5) predict changes in water quality that may result from management actions or changes in land use.…”

Sparrow Modeling to Understand Water-Quality Conditions in Major Regions of the United States: A Featured Collection Introduction1

“…Atmospheric deposition was found to be the largest source of nitrogen in that region and agricultural fertilizer was found to be the largest source of phosphorus. Anning (this issue) describes a model of salinity in the southwestern part of the country (MRB6), which is used to identify the primary sources of salinity to streams, and assess the relative importance of natural geologic materials and human activities related to agricultural water use. Anning finds that although the areal loadings of dissolved solids to streams (load per unit area) from agricultural lands are more than an order of magnitude higher than the loadings from geologic materials, the relative contributions of dissolved solids to streams, on average, from natural geologic materials and agricultural sources are nearly equal across the region.…”