BACKGROUND

Hvdrologlc and geologic setting.

The combined length of the Puerco River and Little Colorado River below the Puerco River (lower Little Colorado River) extends about 318 mi across the southern part of the Colorado Plateau from the Chuska and Zuni Mountains in New Hexlco to the Colorado River in Arizona (fig. 1). The rivers flow past Gallup, New Mexico, and Holbrook and Vinslow, Arizona; Petrified Forest and Uupatki National Monuments; and the Navajo Indian Reservation. The Little Colorado River joins the Colorado River in Grand Canyon National Park, Arizona.

The Puerco River and Little Colorado River are primarily ephemeral streams (fig. 3). Runoff, most of which results from summer thunderstorms or spring snowmelt, recharges the alluvial aquifer beneath and adjacent to the channels. The alluvial aquifer is the principal water supply for domestic, municipal and agricultural uses near the rivers.

The drainage basins are geologically complex and the regional stratigraphic sequence of sedimentary rocks is structurally deformed. This deformation and subsequent erosion has resulted in the local exposure or near-surface occurrence of uranium-bearing strata (Hackman and Olson, 1977). Major uranium deposits occur in the Westwater Canyon Member of the Jurassic Morrison Formation east of Gallup, New Mexico. Radionuclides also occur in the Petrified Forest Member of the Chinle Formation, which has economically viable concentrations of uranium near Cameron (fig. 1).

Figure 3.--The Puerco River is an ephemeral stream in northwestern New Mexico that has been contaminated by radionuclides and other trace elements. The contaminants are mostly sorbed in channel sediments.

Uranium mining in the Little Colorado River basin began in the 1950's and was concentrated near the headwaters of the Puerco River northeast of Gallup in New Mexico (Grants Mineral Belt, fig. 1), and within 50 miles southeast of the Grand Canyon (Cameron Uranium Mining Belt). Mills for processing the uranium ore were operated near both mining areas.

Contamination In the Puerco River basin.

Discharge from mine dewatering upstream from Gallup created perennial flow in the otherwise ephemeral channel of the Puerco River near Gallup, New Mexico to Chambers, Arizona, from the late 1950's to the early 1960's, and again when the nines were in operation from 1969 to 1986. Permits under the National Pollutant Discharge Elimination System (NPDES) were required when the mines reopened in 1969 in an attempt to control the level of radioactive material in effluent water. The activity of radium226 allowed under NPDES was 10 picocuries per liter (pCi/L), or twice the present EPA maximum contaminant level (MCL). At least 63 violations of NPDES permits from mine dewatering occurred between 1980 and 1983 alone (Shuey, 1982). The temporal relation of gross alpha and gross beta activities measured at the Puerco River at Chambers is shown in figure U.

Additional discharges of radionuclides to the Puerco River occurred after a tailings-pond dam failed at the United Nuclear Corporation's Church Rock Mill on July 16, 1979 (fig. 2) (Millard and others, 1984; Shuey, 1982; Ueimer and others, 1981). An estimated 1,100 tons of tailings and 94,000,000 gallons of liquid were discharged to the Puerco River. On the day of the spill, total radionuclide activities in

Figure 4.--Gross alpha and gross beta activities in the Puerco River at Chambers, Arizona. Arizona maximum allowable limits are IS plcocuries per liter of gross alpha minus uranium and 30 plcocuries per liter of gross alpha plus gross beta. The gross alpha activities are not corrected for uranium activity.

the Puerco River at sites in Sew Mexico were £.910 pCI/L of uranium. 12,000 pCi/L of thoriu«-230. 1.0 pCi/L of rmdiiae-226. 2«0 pCi/L of load-210. and 38 pCi/L of polrcium-210 (Hillard and others. 19S-i>. the maximum gross alpha activity ir. the Puerco livtr vas 130.000 pCi/L near the Church Rock Hill and 26.000 to 40.000 pCi/L downstream at Gallup (Shuey. 19S2). During a flood 7 years later, gross alpha activities of 1,700 to 2,200 pCi/L were measured in the Puerco River at Lupton, Arizona (Shuey, 1986).

Trace-element concentrations also exceeded llTlim allowable limits of the State of Arizona (HALS) for surface water in 11 samples collected in 1985 and 1986 (Arizona Department of Health Services, 1986a). Concentrations of total arsenic, copper, manganese, lead, and dissolved seleniua at five sites in Arizona were above the HALS of Arizona in samples taken in February, April, and June 1986 (ADHS, 1986a). Chris Shuey (Southwest Research and Information Center, written commun. , 1987) measured concentrations of arsenic, copper, and lead that exceeded HALS of Arizona in July 1986.

Webb and others (1987a and b) evaluated the quality of water from 14 wells in the alluvial aquifer of the Puerco River basin from the State line of Arizona and New Hexico to Petrified Forest National Monument. The concentrations of gross alpha activity minus uranium equaled or exceeded the MCL of 15 pCi/L for Arizona MCL of 15 pCi/L in five of the fourteen wells. The concentration of radium-226 plus radium-228 exceeded the U.S. Environmental Protection Agency primary MCL of 5 pCi/L in one veil. The concentration of uranium exceeded a State of Arizona recommended limit of 0.035 ntg/L in two wells. The source of contaminants was unknown. Samples of channel sediments and grasses growing near the channel indicated no unusual activities of radionuclides in 1986 (Webb and others, 1987a).