ABSTRACT

Southern New Mexico is characterized as semi-arid region where wastewater reclamation and reuse for irrigation has become important part of water resources planning. This has occurred as a result of the increasing fresh water scarcity, high nutrients in wastewater, and the high cost of advanced treatment required for other wastewater uses. United Nations Millennium Development Goal also targets the use of wastewater as irrigation to reduce the water deficit [1]. Certain quality criteria should be met prior to using wastewater for irrigation. Some of the parameters requiring close attention are electrical conductivity (EC), total dissolved solids (TDS), sodium adsorption ratio (SAR), suspended heavy metals and organic matter (OM). Without proper management, wastewater application

can pose serious risks to human health and the environment [1]. Treatment of urban and industrial wastewater is complex, expensive, and requires energy and technology. The safe disposal of the treated wastewater is also a challenge because the effect of wastewater on the soil and plant environment is complex and depends upon the amount of various elements present in the wastewater. Reuse of 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. Wastewater can add nutrients to the soil system stimulating plant growth, increasing plant NO3

tion. A major objective of land application systems is to allow the physical, chemical, and biological properties of the soil-plant environment to assimilate wastewater constituents without adversely affecting beneficial soil properties [2]. However, when wastewater is irrigated beyond the assimilation capacity of the soil-plant system, it can provide a source of readily leachable nutrient or contaminant [3]. Waste water can also affect soil physical properties, including bulk density (BD), drainable porosity (d), soil moisture retention and hydraulic conductivity (Ks). Recent study on the same location reported lower Ks and macroporosity in the wastewater irrigated areas than in the unirrigated areas [4]. The levels of dissolved OM and suspended solids in effluent depend on the quality of the raw sewage water and the degree of treatment [5,6] Suspended solids present in effluents accumulate in soil voids and physically block water-conducting pores leading to a sharp decline in soil hydraulic properties [4,5]. The reduction in Ks could be due to the retention of OM during infiltration and the change of pore size distribution as a result of expansion or dispersion of soil particles. Application of wastewater with sodium (Na+) content to soil increases sodicity, causes clay dispersion, changes pore geometry, and reduces Ks [7,8]. In contrast, [9] found no adverse impact on the hydraulic parameters while applying standard domestic effluents to soil in Israel. Soils in the arid region are generally calcareous with high pH in the upper soil horizons favoring the precipitation of most heavy metals and reduce the risk of groundwater pollution [10]. The primary goal of land application of wastewater is to maximize vegetative cover to increase the capacity of the soil to serve as a sink for wastewater contaminants, minimize salt accumulation in the root zone, and avoid NO3

- leaching to the groundwater

[11,12]. In this context application of treated wastewater on common arid and semiarid shrubs could be more economical and environmentally beneficial. Soil chemical properties are one of the most researched aspects of wastewater irrigation. Changes due to irrigation vary greatly and are largely dependent on the quality of the irrigation water. However, little work has been conducted on the impact of wastewater irrigation in Chihuahuan desert ecosystem on the native vegetation. An earlier study conducted on part of the West Mesa irrigated site reported that the sprinkler distribution uniformity was low (53.7%) and could have caused the variability in soil chemical and physical properties between canopies and intercanopy areas [11]. In spite of the variability of application, the previous study did not report statistical differences in chemical and physical properties between vegetation canopies and intercanopy areas likely due to low sample size. Similarly, NO3

- and OM content of wastewater listed by the Environmental Protection Agency (EPA) as a method of recycling nutrients and OM were not addressed in that study. The present study overcomes these limitations of the earlier study and provides a detailed account of the impact of wastewater on physical and chemical properties under different vegetation canopies and intercanopy areas within the entire irrigated site. The objectives of this study were to: (1) determine the influence of lagoon treated wastewater interception by shrub canopies on physical and chemical properties of canopy soil, and (2) compare physical and chemical properties among the canopy and intercanopy areas.