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Agricultural drainage water management in arid and semi-arid areas 33Chapter 4Water quality concerns in drainagewater managementINTRODUCTIONThere are several factors to consider when determining the opportunities for and constraints onthe safe use, treatment and disposal of agricultural drainage water. Information and data desiredat the site of drainage water production include: rate of drainage water production per unit area,chemical concentration of constituents of concern, and the rates of mass emission. Drainagewater management requires additional information and data on drainage water quality and itssuitability for the intended water uses as well as an understanding of environmental and healthconcerns. Upstream drainage water management affects the needs and water qualityrequirements of downstream water users. As reliable references are already available onestimating drainage water production volumes (Smedema and Rycroft, 1988; Skaggs and VanSchilfgaarde, 1999), this chapter concentrates on the main factors affecting drainage waterquality and water quality needs and concerns for other water users and incorporates previouswork done by FAO (1997c).DRAINAGE WATER QUALITYTable 7 provides a summary ofchanges in water quality expectedin irrigation return flow relative toirrigation water applied (Tanji etal., 1977). The expecteddifferences in quality in the returnflow are described relative to thesupply water because actualconcentrations in supply watersvary. The operational spill waters(bypass water) from distributionconveyances are not expected todiffer much from the quality of thesupply water except for somepickup or deposition of sediments.In contrast, surface runoff orirrigation tailwater tends to pickup considerable amounts ofsediments and associatedTABLE 7Expected quality characteristics of irrigation return flow asrelated to applied irrigation watersQuality parametersOperationalspillsIrrigationtailwaterSubsurfacedrainageGeneral quality 0 + ++Salinity 0 0, + ++Nitrogen 0 0, +, ++ ++, +Oxygen demandingorganics0 +, 0 0, -, --Sediments +, - ++ --Pesticide residues 0 ++ 0, -, +Phosphorus 0, + ++ 0, -, +0 not expected to be much different than the supply water.+, - slight increase/pickup or decrease/deposition expected.++ expected to be significantly higher due to concentrating effects, application ofagricultural chemicals, erosional losses, pickup of natural geochemical sources,etc.-- expected to be significantly lower due to filtration, fixation, microbiologicaldegradation, etc.Source: Tanji et al., 1977.nutrients, phosphorus in particular, as well as water-applied agricultural chemicals such aspesticides and nitrogen fertilizers (especially anhydrous ammonia). Tailwater is typically similarto the applied waters in salinity and oxygen demanding organics, termed biochemical oxygen
34Water quality concerns in drainage water managementdemand (BOD). Subsurface drainage is enriched in soluble components such as dissolved mineralsalts and nitrates, very low in sediments, whereas other quality parameters are similar to theirrigation water. These changes in water quality of irrigation return flow depend on a number offactors including irrigation application methods, soil properties and conditions, application ofagricultural chemicals, hydrogeology, drainage system, climate, and farmers’ water management.In many regions of the world, municipalities and industries discharge wastewater into opendrains initially intended for the conveyance of only agricultural drainage and storm water. Indeveloping countries especially, municipal and industrial wastewater is often insufficiently treatedbefore disposal into such open drains. The result is a risk that agricultural drainage water qualitymight be seriously contaminated with microbes, pathogens, toxic organics and trace elementsincluding heavy metals.A knowledge of the composition of the drainage effluent and the ability to predict changes inthe composition as a result of changes in crop, irrigation or drainage water management practicesare important in the planning and management of drainage water.FACTORS AFFECTING DRAINAGE WATER QUALITYGeology and hydrologyThe geology of the region plays an important role in drainage water quality. Through weatheringprocesses, the types of rocks (both primary and sedimentary) in the upper and lower stratadefine the types and quantities of soluble constituents found in the irrigated area. The oceanshave submerged many parts of the continents during a period in their geological history. Theuplift of these submerged geological formations and receding seas have left marine evaporitesand sedimentary rocks behind, high in sea salts including sodium, chloride, magnesium, sulphateand boron. These geological formations exist in varying thicknesses, depths and extents on thecontinents. Through hydrological processes, solutes can enter the upper stratum by irrigation orfloodwater, upward groundwater flow in seepage zones, with rising groundwater levels, or capillaryrise. Once the solutes are in the upper strata, they influence the quality of agricultural drainagewater through farmers’ irrigation and drainage water management. The following example showshow the geology and hydrology of an area influence the quality of agricultural drainage water. Italso illustrates the relationship between geomorphology, waterlogging and salinization.Figure 12 shows a schematic crosssectionof the San Joaquin Valley withthe San Joaquin River as the principaldrainage course for this river basin. Theeastern side of the valley was formedfrom the alluvium of the Sierra Nevada,which consists mainly of granitic rocks.The soils derived from Sierran alluviumtend to be coarse textured and nonsaline.The eastern groundwaters arecharacterized as low-salt calciumbicarbonate-typewater with totaldissolved solids (TDS) typically in the200-500 mg/litre range. In contrast, thesoils on the western side were formedFIGURE 12Cross-section of the San Joaquin ValleyCoast RangeMarinesedimentary rocksCaliforniaAqueductvSalt-affectedSan JoaquinRiverNon-salineUnconfined groundwaterConfined groundwaterSierra NevadaIgneousrocksCorcoran clayConsolidated rocks
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