First Place Student Essay The Effect of Acidic Irrigation Water on Soils of the Alamosa River Basin, Colorado Erik T. Holtz* HIS RESEARCH was conducted to determine the impact of PROJECT DESIGN Tacidic irrigation water on the chemical and mineralogi- cal characteristics of the major soil series in the Alamosa Given that the chemistry of the Alamosa River water is River Basin in southwestern Colorado. The Alamosa River acidic and has high levels of sediment and soluble metals, it is used to irrigate about 45,000 acres in the San Luis Valley is suspected that irrigation has greatly altered native proper- of Colorado. The river is impacted by acid in the drainage ties of the soils of the Alamosa River Basin. In order to doc- from the Summitville Mine. Funding was provided by the ument any impact that irrigation with Alamosa River water United States Environmental Protection Agency (USEPA) has had on these soils, a detailed study of the morphologi- to fill data gaps in the USEPA Superfund Site Risk cal, mineralogical, and chemical characteristics of these Assessment and Off-Site Impact Statement associated with soils was conducted across a management gradient. The the Summitville Gold Mine. The study involved three agen- management regimes consisted of virgin sites which had cies: Colorado School of Mines, Golden, Co., AGRO- never been farmed or irrigated, sites which received high- Engineering, Alamosa, Co., and Colorado State University, priority Alamosa River Water, sites which received Rio Ft. Collins, Co. These agencies cooperated under the author- Grande or Conejos River water as an alternate source, and ity of the Colorado Department of Public Health and pre-1984 sites (farmed and irrigated). The pre-1984 sites are Environment (CDPHE) to fulfill the study objectives. The significant because they give a measure of the soil condition research was divided into two phases: Phase l, which includ- before the work of Galactic Resources Ltd at Summitville. ed reconnaissance and intensive characterization of the This report is a summary of extractable copper, cadmium, major soil series in the valley, and Phase 2, which included and zinc data on the primary soil series (Graypoint) and extensive characterization of other minor soil series. extractable copper data for the other minor series in the Basin. The minor soil series investigated were the Mogote series, Lajara series, and the LajaraQuimone complex. Summitville Mine These minor series were sampled in an effort to determine the extent of Alamosa River impacts in the Basin. The Summitville Gold Mine is located in the San Juan Mountains east of the Continental Divide in what is called the Platoro Caldera. This world-class ore bearing body is METHODS compromised almost entirely of the coarsely porphyritic To characterize the chemistry of the soils, chemical South Mountain quartz latite (Bove et al., 1995) and con- analyses were performed on the <2-mm fraction of the sam- tains large deposits of pyrite and enargite which are heavy ples for each management regime (Soil Survey Staff, 1995). metal bearing sulfide minerals. Mining and natural weather- Extractable metals were determined using the AB-DTPA ing processes have resulted in the oxidation of these rock Extraction method (Soltanpour and Schwab, 1977; formations and the subsequent formation of sulfuric acid Soltanpour and Workman, 1979). The AB-DTPA Extraction and soluble metal complexes. In 1984 Galactic Resources method has been shown to give a good index of the amount Limited obtained a permit for a full-scale, open pit, cyanide of elements available to plants (Soltanpour and Schwab, heap-leach mining operation at the site (Pendleton et al., 1977; Soltanpour, 1991). Simultaneous analysis for 40 dif- 1995). Operations continued until Galactic Resources Ltd., ferent elements made on the extraction solutions using an under federal pressure to clean up acid water seeps and other Inductively Coupled Plasma Atomic Emission Spectrometer potential watershed contamination threats, declared bank- (ICP) (Soltanpour et al., 1982). Total metal content was ruptcy in late 1992 (Pendleton et al., 1995). At that time, the determined using EPA Method 3050 digest and ICP analy- USEPA took possession of the mine under emergency sis. response provisions under CERCLA. In 1995, the Summitville Gold Mine was declared a USEPA Superfund Site. RESULTS AND DISCUSSION Graypoint Series Dep. of Soil and Crop Science, C111 Plant Science Building Center Street, pH. It should be noted that pH throughout the Graypoint Colorado State Univ., Fort Collins, CO 80523 ([email protected]. soils irrigated with Alamosa River water was lowered colostate.edu). Received 7 June 1999. (Figures 1, 2, 3). In comparing soils irrigated with Alamosa Published in J. Nat. Resour. Life Sci. Educ. 28:7983 (1999). River water to virgin soils, the difference can be as much as http://www.JNRLSE.org 2 pH units. J. Nat. Resour. Life Sci. Educ., Vol. 28, 1999 79 Fig. 1. Soil pH and total Cu, Cd, and Zn concentrations as a function of depth across the management gradient. Fig. 2. Soil pH and AB-DTPA extractable Cu, Cd, and Zn concentrations as a function of depth across the management gradient. 80 J. Nat. Resour. Life Sci. Educ., Vol. 28, 1999 Total Metal Levels. Results for total Cu, Cd, and Zn Table 1. National baseline metal averages and ranges for soils within concentrations with depth across the management gradient the conterminous United States (Shackellete and Boerngen, 1984; Lindsay, 1979). are displayed in Figure 1. Standard errors are only shown for the surface and first B-horizon because these are the only Element Baseline range Baseline geometric mean horizons for which replicate samples were taken. Sample mg/kg size in the surface and first B-horizon is equal to five or six Cadmium 0.010.7 0.06 depending upon if a duplicate is included in the average. All Copper 30 2100 other data points refer to a value for a single sample from the Zinc 50 1100 main pedon at each site. In the surface horizon of the flood- irrigated Alamosa River site, total Cu and Zn (68.3 mg/kg mg/kg) are found in the surface horizon of the flood-irrigat- and 97.1 mg/kg, respectively) are higher over all other sites. ed Alamosa River site. Extractable levels of Cu, Cd, and Zn Copper concentrations in the subsurface horizons are more are significantly elevated above virgin soil conditions at all elevated in the flood-irrigated Alamosa River site than in all depths in the flood-irrigated Alamosa River site. Extractable other management regimes. The PRE-84 site has more total Cu (14.42 mg/kg) and Zn (3.0 mg/kg) in the surface horizon Cu (48.6 mg/kg) than the virgin (25.01 mg/kg) and the of the PRE-84 site are elevated above the virgin and flood- flood-irrigated Rio Grande River sites (22.1 mg/kg) in the irrigated Rio Grande River sites, indicating that historic con- surface horizon, indicating that Cu has been historically ele- ditions along the Alamosa River were also conducive to vated in Alamosa River flood-irrigated Graypoint soils. enhanced extractability of metals. In the subsurface hori- Total Zn in the PRE84 surface horizon (77.7 mg/kg) is not zons, the results for extractable metals are quite variable. In significantly elevated above the virgin (66.8 mg/kg) or the first B-horizon, both the PRE-84 and flood-irrigated flood-irrigated Rio Grande sites (76.8 mg/kg). Total Cd con- Alamosa River sites have significantly elevated Cu levels centrations across the management gradient are about the over the flood-irrigated Rio Grande River and virgin sites. same at all depths (Figure 1). Total Cu, Cd, and Zn concen- At 48 cm in the flood irrigated, Alamosa River site, trations are not elevated beyond baseline ranges for metals extractable Cu is still elevated beyond all other management in agricultural soils in the conterminous United States for regimes. Extractable Cd is not significantly elevated for any any of the management regimes at all depths (Table 1; site across the management gradient in the subsurface. Shacklette and Boergnen, 1984; Lindsay, 1979). Extractable Zn levels at all depths in the flood-irrigated Extractable Metal Levels. Results for AB-DTPA Alamosa River site are elevated above virgin and flood-irri- extractable Cu, Cd, and Zn with depth across the manage- gated Rio Grande sites. ment gradient are displayed in Figure 2. The highest con- Ratio of AB-DTPA Extractable Metals to Total centrations found across the management gradient for Metals. Another way of looking at the effect of Alamosa extractable Cu (3 6.7 mg/kg), Cd (0. 18 mg/kg), and Zn (9.8 River water on metal solubility in the Graypoint soil series Fig. 3. Soil pH and the ratio of total to extractable Cu, Cd, and Zn concentrations as a function of depth across the management gradient. J. Nat. Resour. Life Sci. Educ., Vol. 28, 1999 81 Fig. 4. AB-DTPA extractable Cu in the A-horizon of the Mogote soil Fig. 6. AB-DTPA extractable Cu in the A-horizon of the La Jara soil series as a function of irrigation management. series as a function of irrigation management. minor soil series relative to the three different management regimes is shown in Figures 4, 5, and 6. With the exception of Figure 6, there appears a definite trend linking irrigation with Alamosa river water to higher extractable copper lev- els. In Figure 6, the virgin site appears to have the same amounts of extractable copper as the flood Alamosa site. The difference found between the flood Alamosa site and the flood Conejos site is more consistent with the phase I findings, which leads us to believe that the virgin site is not undisturbed.