Land Application of Industrial Effluent on a Chihuahuan Desert Ecosystem: Impact on Soil Physical and Hydraulic Properties

Land application of treated industrial effluent could be beneficial especially in areas where water stress is a major concern primarily due to limited water resources, higher water demands, and limited economic resources. The primary objectives of this study were to: (1) determine the influence of lagoon treated wastewater on physical and chemical properties of soil in canopy and intercanopy areas, (2) compare soil physical and chemical properties in the irrigated and unirrigated plots, (3) determine the variability of soil physical and chemical properties, and (4) identify the minimum number of principal components (PCs) necessary to explain the total variability in soil physical and chemical properties. The West Mesa Industrial Park near Las Cruces, New Mexico USA has applied lagoon treated industrial effluent since 2002 to 36-ha of Chihuahuan Desert native vegetation (mesquite and creosote) by a fixed-head sprinkler irrigation system. Core and bulk soil sample were collected from under mesquite and creosote canopies and intercanopy areas from two irrigated plots and one unirrigated plot.

From 2002 to 2007, the average sodium adsorption ration (SAR) of irrigation water was 32.97, electrical conductivity (EC) 3.90 dS m-1, and pH 9.7. The sprinkler uniformity coefficient for both irrigated plots was low and soil EC measured at the end of the irrigation uniformity tests showed some correlation with the treated wastewater distribution. More water was collected under the canopies (116.62 ± 5.18 cm3) than the intercanopy areas (82.55 ± 5.87 cm3). In general, soil physical properties, including bulk density (BD), sand, silt, and clay contents did not show any significant effect of treated wastewater application. The saturated hydraulic conductivity (Ks) and drainable porosity (Θd) were lower and the available water content (AWC) was higher at 0-20 cm depth of the irrigated plots than unirrigated plot. Higher values of chloride (Cl-) under creosote canopies at 100-150 cm depth than mesquite canopies and intercanopy areas of the irrigated plots were due to higher canopy interception of sprinkler sprays and deeper leaching of the solute and could be a source of groundwater pollution.

In irrigated plots at 0-20 cm depth, pH (>9) and Na+ (>693 mg kg-1) were higher in the intercanopy areas than under the vegetation canopies. Variability in soil properties identified by coefficient of variation (CV) ranked NO3- (CV=0.65), Cl- (0.65); SAR (0.47), Ks (0.41), Na+ (0.38), ESP (0.38) and EC (0.37) as most variable; silt (0.32), AWC (0.20), field capacity (FC; 0.16) and organic matter (OM; 0.17) as moderately variable; and sand (0.01), clay (0.08), BD (0.03) and pH (0.03) as least variable. The PCA grouped soil properties into five distinct PCs: soil salinity, soil sodicity, water transmission, soil texture, and water storage based on the attributes present in each one of them. Overall, compared to the unirrigated area, the salinity and sodicity in the irrigated areas had increased more than one order of magnitude at 0-20 cm depth. The PCs composed of the variables associated with soil salinity and sodicity explained a large variability of the measured attributes. Since these indicators are directly associated with the chemical properties of treated wastewater, there is a need to initiate efforts to reduce the chloride and sodium concentrations of the applied treated wastewater. Deep rooted mesquite and creosote bushes are the primary vegetation in the study area. The majority of the mesquite roots are usually distributed within top 100 cm depth and creosote within the top 25 cm. Average SAR for 100 cm depth under canopies was 18.46 ± 2.56 and could threaten the survival of woody and especially the perennial herbaceous plant species growing in the study area.