Lake Powell Coring

Science Center Objects

In response to the August 5, 2015, Gold King Mine Spill from the Bonita Peak Mining District that resulted in the release of three-million gallons of mine-impacted waters, the Utah Water Science Center, in partnership with the Utah Division of Water Quality, National Park Service, and U.S. Bureau of Reclamation, cored the San Juan and Colorado River deltas in multiple locations in Lake Powell, Utah during October and November, 2018. Coring took place from a barge along longitudinal transects in both the San Juan and the Colorado River deltas. For the mose part, continuous cores of deltaic sediment terminated either in pre-Lake Powell alluvial deposits or sandstone bedrock of the Glen Canyon Group, which ensured that the deltaic sediments had been drilled through their full thickness. 

Background: Lake Powell began to fill with water on March 13, 1963 and reached full capacity more than 17 years later on June 22, 1980.  By September of 1986 the reservoir also accumulated more than a cubic kilometer of sediment (868, 231 acre-feet), equivalent to 3.2% of the reservoir capacity (Ferrari, 1988).  Repeat bathymetric surveys have shown that approximately the same volume of sediment (1 km3) was remobilized during the drought period 1999–2005 as lake level fell and deltaic sediment was eroded (Pratson et al., 2008).

Recently, the August 5, 2015, Gold King Mine (GKM) spill from the Bonita Peak Mining District into the upper Animas River, a tributary to the San Juan River, has brought significant attention to the transport and fate of sediments in this watershed.  This event released 3 million gallons of mine impacted water, and transported 490,000 kg (~540 tons) of metals, the vast majority of which were transported to Lake Powell as fine grained sediment by August 2016 (U.S. EPA, 2016).  In addition to persistent discharge of mine waste, events similar to the GKM release occurred in the 1970s, and it is generally assumed that the ultimate repository for toxic metals sourced from the mining districts in the upper Animas watershed is the fine grained deltaic sediments in the San Juan Arm of Lake Powell (U.S. BOR, 2015).

Map of Lake Powell study area.

General Lake Powell study area.

While some work on deltaic sediment has been done, very little is known about the concentration, distribution, and bioavailability of metals through the total thickness the deposits.  Furthermore, San Juan River deposits accumulated prior to completion of Glen Canyon Dam would have also contained heavy metals related to historic mining in the Upper Animas watershed.  Such deposits are known to exist in Lake Powell, and in places their thickness is likely greater than that of the overlying deltaic sediment (cf. Miser, 1924).  The potential for these pre-1963 deposits to contain significant amounts of bioavailable metals is high, yet they remain uncharacterized.  Retrieving and analyzing sediment from deltaic deposits and underlying alluvial sediment is entirely tractable; however, it requires commitment of resources and effort not yet afforded to the issue.

Existing sediment data gives cause for concern.  Short cores recovered from the San Juan delta in August of 2010 show a measureable increase in metals at ~4 m depth (Hornewer, 2014).  This shift may be related to cessation of the remediation activities in the upper Animas that took place during the late 1990s and early 2000s (Church et al., 2007; Runkel et al., 2009), or to shifting patterns of fine grained sedimentation driven by rising lake level beginning in 2005 (cf. Hem, 1985).  Given sedimentation rates in the San Juan delta (8–180 cm/year measured by our sediment traps from 2015–present), correlation of this increase with either scenario is plausible.  While the state of Lake Powell’s deltas is essentially unknown, they continue accumulating and redistributing potentially toxic concentrations of numerous metals (especially As, Cd, Cu, Hg, Pb, Se, and Zn).  Understanding the total mass, spatial distribution and bioavailability of these metals is critical for understanding risks posed to water quality in Lake Powell, and these factors need to be thoroughly investigated.

Objectives and Scope: The objective of the study is to collect and characterize sediment through the full thickness of the San Juan delta.  These efforts began with the retrieval of cores from multiple locations. Total cores have been scanned. Work on the cores at the National Laustrine Core Facility will continue for some time. Updates and reports will be posted as they become available.

Chemical analyses of the sediment will help determine both the character of metal deposition and the chronology of sedimentation, resulting in rates of metal deposition in Lake Powell.  This information will contextualize the relationship of historical metal deposition to mining and remediation activities in the Upper Animas Watershed, Colorado (presumed source of most toxic metals deposited in the San Juan Arm of Lake Powell).  The results will also be useful to assess the potential water quality impacts resulting from lower lake levels and remobilization of deltaic sediment.  Under an expanded scope, a parallel study of the Colorado River delta would provide an important comparison to the San Juan delta and fully characterize water quality issues related to deltaic sediment.

Relevance:  Lake Powell is the nexus for water quality in the southwestern United States.  Freshwater resources of the Upper Colorado River Basin supply water to Lake Powell which conveys this resource to the Colorado River in the Grand Canyon, Lake Mead, and ultimately supports a wide range of agricultural and culinary uses in Arizona, Nevada, and California.  Of immediate concern to the state of Utah is the impact of water quality for both recreational activities on Lake Powell and its future use as a large scale municipal water supply. 

Lake Powell is one of the premier water-based recreation destinations in the world, and it is a Blue Ribbon sport fishery.  Since 2012 the State of Utah has had a fish consumption advisory due to mercury (Hg) levels in specific fish populations (Utah Department of Health, 2017).  In addition, Utah Department of Environmental Quality (DEQ) has identified exceedances of water quality criteria for cadmium (Cd) at the sediment water interface in both the San Juan and Colorado arms of the lake.  However, due to limited samples Lake Powell has not been placed on Utah’s 303(d) list of impaired waters for Cd.  Similarly, it is known that downstream of Lake Powell on the Colorado River there is mercury and selenium (Se) bioaccumulation in the food web with highest concentrations in basal resources just below Glen Canyon Dam, suggesting export of bioavailable sources from Lake Powell (Walters et al., 2015).

The planned use of Lake Powell as a water resource for the rapidly growing community of Saint George, Utah via a 139 mile water supply pipeline (WCWCD, 2017) is also strong motivation for understanding historical, current, and future water quality issues in Lake Powell.  Retrieval and analysis of sediment cores through the entire thickness of the San Juan delta is essential to understanding both historical and future water quality issues which are inextricably linked by large changes in lake level and attendant remobilization of deltaic sediment.  The Hg advisory issued in the years after massive remobilization of deltaic sediment during the 1999–2005 drought hints at linkages between sediment and water quality in Lake Powell.  Given an uncertain climatic future, growing populations in the western U.S., and proposals to lower the operating level of Lake Powell (Schmidt et al., 2016) concern about the nature of deltaic deposits is warranted.