Australian. Water Conservation and Reuse Research Program

Transcription

1 Australian Water Conservation and Reuse Research Program Impacts on soil, groundwater and surface water from continued irrigation of food and turf crops with water reclaimed from sewage. Daryl Stevens 3, Murray Unkovich 1, Jim Kelly 3, Guang- Gou Ying 2. Project Leader Daryl Stevens 1 Soil and Land Systems, Adelaide University 2 CSIRO Land and Water 3 ARRIS Pty Ltd. January 2004 ISBN

2 Impacts on soil, groundwater and surface water from continued irrigation of food and turf crops with water reclaimed from sewage. This report was supported by United Water International and prepared for the Australian Water Conservation and Reuse Research Program Disclaimer This review is presented as is without any warrantees or assurances. Whilst all reasonable efforts have been made to ensure the information provided in this review is current and reliable, ARRIS Pty Ltd and the contributors of this work cannot accept any responsibility for inconvenience, material loss or financial loss resulting from this review. We do not accept any responsibility for errors or omissions in the contents, however they may arise. ARRIS Pty Ltd and contributors may identify products by proprietary or trade names to help readers identify particular types of products. We do not endorse or recommend the products of any manufacturer referred to in this review. Other products may perform as well or better than those specifically referred to in this review. ii

3 In some cases the land is so well drained naturally that irrigation can thus be continued for a very long time. However, far more typically, the process of groundwater rise and salt accumulation proceeds so inexorably, that sooner or later the provision of artificial drainage becomes essential (Hillel 2000) iii

4 Executive Summary Water can be reclaimed from sewage effluent and used for irrigation, reducing the level of pollutants entering water bodies, by filtering it though soils and plants. Reclaimed water typically contains high levels of nutrients (mostly NH + 2-4, PO 4 and organic nutrients), salts (particularly Na +, K +, Cl - ), bacteria, viruses and parasites. Chemical constituents may pose longterm problems, if not managed appropriately, through accumulation in soils and food chain transfer, or contamination of water bodies. However, the micro organisms in reclaimed water typically die off relatively quickly, once removed from their hosts, and do not pose a long-term threat to soil or human health. Unlike water, the soil is effectively a non-renewable resource. The biggest problem will continue to be the management of salt, as it is for most irrigation systems. Recycling salt to the sea would be a good option, provided other unwanted components are removed first, or are at low concentration. Plant nutrients can be utilised for productive purposes and organic components broken down to the smallest molecules possible (preferably CO 2 which can be refixed by plants). For humans, being at the top of the food chain has the inevitable disadvantage of accumulation of pollutants in our bodies. While this cannot be completely prevented, it can be minimised with careful attention to maintaining the lowest acceptable levels of contaminants in our soil and water resources (from a human and environmental health aspect and practical sense), which support the crop and forage plants that provide the avenues for toxic compounds to enter the food chain. It is important to note that reclaimed water is not the only source of pollutants and an assessment of environmental threats must always take into account all sources of pollutants. With the agricultural use of reclaimed water, management of drainage quality and quantity will undoubtedly be the most important challenge to meet in the long term. As Hillel (2000) pointed out, irrigation can be made sustainable, but at a cost. We believe that reclaimed water irrigation can also be made sustainable, at a cost. But it will require continued monitoring of many of the potential issues identified in this paper, and monitoring for unforeseen issues that may also arise due to changing circumstances. It is inevitable that water bodies will still become sinks for some of the water from reclaimed water irrigation systems, and the compounds contained therein. Since even the best planned schemes are not able to predict and manage all nuances of a reclaimed water system or landscape, monitoring of receiving water bodies will be essential to determine changes in water quality. This monitoring provides feedback to reclaimed water uses and regulators, so that management changes can be made to minimise any off-site impacts. Irrigators will thus need to work closely with natural resource managers. Some problems which beset irrigation systems (salinity, nitrate leaching, sodicity) do not discriminate between reclaimed and other water sources and these will continue to be potential issues regardless of water source. Reclaimed water will increase in value with time as other supplies of water dwindle relative to demand. The possibility of augmenting potable supplies through indirect recharge, or via substitution of reclaimed water for irrigation, which frees up higher quality groundwater for potable uses, should be seen as real opportunities. Provided that the issues raised above are managed successfully by the industry, reclamation and reuse of water from sewage effluent is likely to be seen as an essential (perhaps mandatory) component of well managed, environmentally sustainable cities of the future. Given the number of reclaimed water irrigation schemes operating successfully in the world, it is clear that such systems can be made safe in terms of pathogenic organisms and (at least) short to medium-term human health. The industry has thus moved successfully from theory into practice. iv

5 The next steps for the industry will be facing the potential long-term environmental problems highlighted in this review. For the industry to remain viable in the future the following research areas might prove valuable: a more thorough exposé of the comparative quality of reclaimed vs other irrigation waters; better risk assessments for organic toxicants in the environment which will require more studies on the types of toxicants and their sources in the wider environment; better tools for estimating the capacity of irrigated soils to mineralise N for prediction of nitrate leaching risk, and the possibilities for using perennial catch or cash crops to retrieve nitrate; development of practical tools for growers for the monitoring of nitrate leaching and soil sodicity. the development of appropriate thresholds, based on a measure of bioavailabilty, for inorganic contaminants in soils, particularly heavy metals and metalloids; better assessment of the relative boron tolerance of crop species; and salinity management will remain an ongoing challenge v

6 Table of contents Executive Summary iv List of Tables vii List of figures vii Abbreviations viii Definition viii Introduction 1 Hydrologic cycle - evapotranspiration, recharge and run-off 1 Sustainability 2 Dual objectives of a reclaimed water irrigation system 2 Differences between reclaimed water and other irrigation water sources 3 Water use and loadings 5 Irrigating with reclaimed water: Impacts on the soil resource 7 Salinity 8 Crop response to soil salinity 8 Managing root zone salinity 11 Sodicity 14 Theory 14 Practice 17 Accumulation of toxic compounds 20 Macronutrients 20 Heavy metals and metalloids 20 Boron 25 Organic contaminants 28 Irrigating with reclaimed water: impacts on water resources 30 Leaching of nitrate 30 Leaching of salts 32 Leaching of other inorganic elements 32 Leaching of organic compounds 32 Surface transport of phosphorus, suspended solids and organics. 33 Groundwater recharge 33 Management options for soil resource protection 35 Minimising problems using irrigation technology 35 Opportunities and problems over the next 10 years 37 Conclusions 37 References 39 vi

7 List of Tables Table 1 Constituents of concern in wastewaters for protection of soil and water resources... 3 Table 2 Key issues for irrigation water quality in relation to soil and water resources... 3 Table 3 Quality of Class A reclaimed water (CARW) and two major groundwater aquifers on the Northern Adelaide Plains, South Australia... 4 Table 4 Characteristics of raw wastewater, treated wastewater and control (bore) irrigation water from a study in Morocco... 4 Table 5 Average salinity at selected sites in the Murray River in South Australia Table 6 Parameters to consider when determining reclaimed water irrigation needs... 5 Table 7 Approximate nutrient uptake in vegetable crops... 7 Table 8 Approximate values for soil saturated hydraulic conductivity and impacts on utility... 8 Table 9 Average root zone salinity tolerance of vegetable and fruit crops, threshold irrigation water salinities before yield loss as a function of soil type, and % yield loss/ds/m after threshold is reached Table 10 Approximate soil water storage for a range of textural classes Table 11 Approximate rooting depths (m) for fruit and vegetable crops grown under irrigated or dryland conditions Table 12 Potential sodicity hazards from irrigation waters of varying electrical conductivity (EC iw ) Table 14 Soil Contaminant Investigation Levels and Loadings (mg/kg) Table 15 Maximum level of metal contaminant in food...24 Table 16 Boron concentration (mg/l) in reclaimed and other irrigation waters Table 17 Maximum boron concentrations in irrigation or soil water tolerated by a variety of crops, without reduction in yields Table 18 Irrigation method and impact of saline irrigation waters on crops List of figures Figure 1 Relationship between amount of irrigation water applied (mm), concentration of inorganic compounds (mg/l) and elemental load (kg/ha). The approximate range of concentrations for some reclaimed water constituents are indicated on the right, and the approximate irrigation water needs of a range of crops shown underneath. The figure provides a guide to the relative loadings of nutrients, heavy metals and salts for different types of crops. For example, for a crop of potatoes which would typically receive some mm irrigation water would add more than 1t/ha of salt, <100 kg/ha of nitrogen and <10kg/ha boron. Much larger loadings would be anticipated for a citrus crop and about half for say tomatoes... 6 Figure 2 Changes in soil electrical conductivity (EC 1:5 ) with depth related to historical water use. EC 1:5 scale is a natural log transformed because data were not normally distributed. The red line indicates the level that salinity is likely to affect many horticultural crops (from Stevens et al. 2003b) EC se for these data are EC 1:5 x 8.2 (Kelly et al. 2001). The red vertical line indicates the salinity (EC se 3 ds/m) at which crop yield might begin to be depressed Figure 3 Potential soil salinity and sodicity hazards based on 1:5 soil:water extracts Figure 4 Relationship between SAR and EC of irrigation water and likelihood of soil structure breakdown (from ANZECC and ARMCANZ 2000) Figure 5 Soil salinity(a) and ESP(B) before and after irrigation with reclaimed water (from Jayawardane et al. 2001) Figure 6 Change in soil boron concentration for soils irrigated with three types of water on the Northern Adelaide Plains. Note: Soil boron concentration is a log scale. The red line indicates toxic yield threshold above which yield reduction begins to occur. Source (Stevens et al. 2003b) vii

8 Abbreviations EC electrical conductivity EC se electrical conductivity of a saturated paste soil extract EC iw electrical conductivity of irrigation water DUL - drained upper limit Definition Reclaimed water water that has been derived from sewerage systems of industry processes and treated to a standard that is appropriate for its intended use (EPA Victoria 2003). viii

9 Introduction Hydrologic cycle - evapotranspiration, recharge and run-off Water is one of the key elements that sustains all life on earth. Land plants access this water through the soil, but the rooting zone holds <1% of the globes water resources, the bulk being in the oceans (97%) and in ice (approximately 2%). Fortunately water continually moves freely between these pools via evaporation, condensation, precipitation, runoff and deep drainage, such that water lost from the rooting (vadose) zone by plant transpiration, evaporation or deep drainage, is replaced annually by precipitation and recharge. On a global scale, about 65% of annual precipitation is returned to the atmosphere via evapotranspiration, the remainder contributing to recharge of aquifers or returned to the sea via runoff and lateral flows. For crops or vegetation systems, these processes can be considered in a water balance. For example, Precipitation (P) = Evaporation (transpiration + evaporation) + Runoff (R) + Deep drainage (D) + change in soil water storage ( S). In the case of irrigated crop systems the term P would include irrigation water. Waters may also carry inorganic (e.g. salts) and organic (e.g. proteins) compounds. While water is able to move freely from one phase to another (solid, liquid, gas), salts and the many other compounds which it carries are not, and thus these can become trapped in soils and plants as water is evaporated/transpired. Water is also able to transport compounds through soil as it moves up or down, and is able to pick up both soluble and insoluble compounds from the soil and move them through the soil profile, or into other water bodies via runoff or deep drainage. Water is thus a major transport pathway for chemicals (natural and anthropogenic) in the environment. Since irrigation systems increase water inputs, they can increase the input and transport of these compounds through the plant-soil-water-atmosphere continuum. By increasing water flux, irrigation systems can thus: increase evapotranspiration; increase deep drainage and recharge; increase runoff (discharge); increase the movement of salts and other compounds through the soil and plant; and transfer salts and other compounds to ground and surface waters All irrigation systems must manage these fluxes of water and its impurities to balance the costs and benefits in the short and long term. As part of the natural hydrologic cycle, most water that is used for domestic and industrial purposes is also eventually discharged to other water bodies via sewage treatment plants. However, this water often also contains a high level of salts and other inorganic and organic compounds. Consequently, attention is being focussed on the careful use of this water to (a) protect fresh and near-shore marine water resources from pollution, and (b) augment irrigation water supplies. The objective of this review is to identify the conditions required to protect soil and water resources during the continued use of reclaimed waters for irrigation of food crops. While the review considers possible off-site effects of reclaimed water systems, it maintains a focus on the on-site management issues in relation to maintaining the sustainability of reclaimed water irrigation. What do we mean by sustainability? 1

10 Sustainability In an agricultural sense, sustainability refers to the ability to undertake a production system without long-term detriment to the land and wider environment. This does not mean that agricultural activity should not modify the land or wider environment, rather that any changes should not compromise ongoing productivity, functionality or diversity of the environment for future generations. In assessing the sustainability of reclaimed water schemes, both on and off site issues need to be considered. Onsite, the scheme must ensure the maintenance and/or improvement of soil fertility and plant health for the foreseeable future (DPIE 1989). Offsite, the scheme must not cause undesirable impacts on the environment. The myriad of issues underpinning sustainable development are captured succinctly in Australia s National Strategy for Ecologically Sustainable Development (Environment Australia 2001), which has three core objectives viz: Enhance individual and community well-being and welfare by following a path of economic development that safeguards the welfare of future generations; Provide for equity within and between generations; and Protect biological diversity and maintain essential ecological processes and life-support systems. Within these core objectives, 21 indicators were nominated to rate Australia s development on an ecologically sustainable basis. Irrigation with reclaimed water could potentially be assessed on 10 of the 21 (i.e. Healthy living; Economic capacity; Industry performance; Economic security; Management of natural resources water; Management of natural resources agriculture; Biodiversity and ecological integrity; Coastal and marine health; Freshwater health; and Land health). This review focuses on the two natural resource indicators (soil and water), but we are aware of the other indicators, which we will mention where there is specific relevance. Dual objectives of a reclaimed water irrigation system Approximately 1,600,000 ML of water and 230,000 dry t of biosolids are produced annually in Australian sewage treatment works (Dillon 2000; Stevens et al. 2003a). During the last 10 years governments have encouraged better use of these wastes and water through irrigation of agricultural crops, effectively turning the waste into a resource. However, there are many challenges when marrying wastewater (safe disposal) objectives with irrigated agricultural production. As the soil effectively acts as a filter for many of the compounds carried in the water, these compounds inevitably build up in the soil. Some of these compounds, such as nitrogen (N) and phosphorus (P) can be usefully taken up by plants, others such as boron (B) and salts (e.g. Cl - ) may be directly phytotoxic, or may cause changes in soil chemistry which impact on crop growth or soil structure. Sewage effluent water typically contains high levels of nutrients (mostly NH 4 +, PO 4 = and organic nutrients) from human and domestic wastes, salts (particularly Na +, K +, Cl - ), and bacteria, viruses and parasites. While the chemical constituents may pose long term problems through accumulation in soils and food chain transfer or contamination of water bodies, the micro organisms typically die off very quickly once away from their hosts and do not pose a long-term threat to soil or human health. We reproduce here (Table 1) part of the table of Asano et al. (1985) listing the constituents of concern for soil and water resource protection in the use of reclaimed water. 2

11 Table 1 Constituents of concern in wastewaters for protection of soil and water resources Constituent Measured parameters Reason for concern Stable organics specific compounds (e.g. pesticides, chlorinated hydrocarbons) tend to resist conventional methods of wastewater treatment some compounds environmental toxins may limit suitability for crop irrigation Nutrients Nitrogen (N) Phosphorus (P) Potassium (K) essential for plant growth and can enhance value of water for crop irrigation when discharged to the aquatic environment N and P can lead to algal blooms, anaerobic conditions and fish deaths pollution of groundwaters Hydrogen ion activity ph affects metal solubility Heavy metals Cadmium, zinc, nickel, mercury, arsenic, copper accumulate in the environment and become toxic to plants, animals, humans Dissolved inorganics Total cations, electrical conductivity, Na, Ca, Mg, Cl, B, sodium absorption ratio excessive salinity damages crops excess sodium may cause soil structural problems (from Asano et al. 1985) It is noteworthy that a similar list of issues are highlighted in Australian guidelines (Table 2) for irrigation water quality criteria for crop production (ANZECC and ARMCANZ 2000). Therefore, many of the issues apply to irrigation in general rather than solely to reclaimed water. Table 2 Soil Plants Water resources Important associated factors Key issues for irrigation water quality in relation to soil and water resources Key Issues Root zone salinity, soil structural stability. Build up of contaminants in soil. Release of contaminants from soil to crops and pastures. Yield, salt tolerance, specific ion tolerance, foliar injury, uptake of toxicants. Contamination by pathogens. Deep drainage and leaching below the rootzone. Movement of salts, nutrients and contaminants to groundwaters and surface waters. Quality and seasonality of rainfall. Soil properties, crop and pasture species and management options. Land type, groundwater depth and quality. (from ANZECC and ARMCANZ 2000) Differences between reclaimed water and other irrigation water sources It is worth outlining the principal differences between reclaimed water and other irrigation waters, prior to discussing in detail the use of reclaimed water irrigation and protection of soil and water resources. Generally, reclaimed water tends to have a higher salt content, nutrient load and level of toxic compounds than other irrigation water sources. However, other irrigation waters are not free of these components. For example, in the recent audit of Australia s water resources (NLWRA 2001) it was found that 61% of the river basins examined exceeded nutrient quality standards, 32% exceeded acceptable salinity levels, and 61% exceeded turbidity criteria. Table 3 gives a comparison of reclaimed water and other irrigation waters from the Northern Adelaide Plains in South Australia, while Table 4 has data from a similar comparison in Morocco. In the latter study it can be seen that where fresh surface waters were not available, the levels of salt and nitrate in bore water used for irrigation exceeded that of reclaimed water. 3

12 Table 3 Quality of Class A reclaimed water (CARW) and two major groundwater aquifers on the Northern Adelaide Plains, South Australia CARW T1 Aquifer T2 Aquifer Parameter Unit Average Min Max Min Max ph TDS mg/l EC (calc.) ds/m Total N mg/l Total P mg/l E.coli /100ml 0 a na Na na na SAR Cl mg/l a is median value; na indicates not analysed (from Kelly et al. 2001) Table 4 Characteristics of raw wastewater, treated wastewater and control (bore) irrigation water from a study in Morocco Parameter Raw Wastewater Treated Wastewater Bore water (control) ph EC (ds/m) P-PO 4 (mg/l) N-NH 4 (mg/l) N-NO 3 (mg/l) HCO (meq/l) SO 4 (meq/l) Cl (meq/l) Ca (meq/l) Mg (meq/l) K (meq/l) Na (meq/l) SAR Values are mean of 48 samples (from Hamouri et al. 1996) Although about 50% of nitrogen (N) and 60% of phosphorus (P) are removed from sewage during treatment (Bahri 1998) reclaimed water remains very much higher in N and P (two important plant nutrients) than other potential irrigation waters. For other parameters, there is great variability within fresh irrigation waters, being either of greater or lesser quality than reclaimed water. However, there is a strong tendency for reclaimed water to have a higher salinity and higher concentrations of sodium (Na) relative to other cations than other irrigation waters. This salinity originates principally from domestic and industrial water softeners (Asano et al. 1985) and through evaporative concentration during consecutive lagooning treatments (Marecos do Momonte et al. 1996). In the U.S. highly saline (>8 ds/m) agricultural drainage waters are reused for irrigation (Ayars et al. 1993). Walker et al. (2002) report salinity levels in groundwater used for irrigation in South and Western Australia to be ds/m. While in South Australia, Cugley et al. (2002) reports that over a ten year period, average salinity in the Murray River increased from 0.34 to 0.60 ds/m as one moved downstream (Table 5). 4

13 Table 5 Average salinity at selected sites in the Murray River in South Australia Site location Mean salinity Conductivity ds/m Total Dissolved Salts (mg/l) Lock Morgan Mannum Murray Bridge Tailem Bend (from Cugley et al. 2002) Some irrigation waters may also be very high in other elements. In the US, agricultural drainage waters used for irrigation may contain some 5-7 mg/l boron (Keren and Bingham 1985), while in Chile, river water used for irrigation water has some 17 mg/l boron (Ferreyra et al. 1997). Therefore management of excess salts and toxicants is not unique to reclaimed water irrigation systems, however, the high N and P nutrient loads are. The management of recharge, run-off and salinity are issues for all irrigation water systems. The critical issues in any reclaimed water irrigation scheme will vary depending on the constituents of the water. For example, there may be a ten fold range in the nutrient, salinity, sodicity, or boron concentrations in both reclaimed and other irrigation waters, depending on the source water and sewage treatment imposed. Thus in some cases reclaimed water salinity may be less of a problem than for other irrigation waters, and boron or cationic balance (sodicity) may be more critical. While in other cases salinity may be the critical issue. Each reuse scheme must be assessed on site specific parameters. However, there are some basic principles that can be applied in assessing the feasibility of a reclaimed water irrigation scheme. Water use and loadings Since the soil is the ultimate repository for most of the substances contained in reclaimed water, it is necessary to estimate the amount of the compounds (load) applied to the land in the water, prior to assessing the capacity of soils to retain those compounds without detriment. For irrigation management, the amount of water applied is calculated as a function of crop evaporative demand (ET), the capacity of the soil to store water (total and plant available), likely rainfall inputs, and the hydraulic conductivity of the soil (leaching rate). A leaching fraction is required in all irrigation systems, regardless of water quality, to remove the inevitable build up of salts from the rooting zone (Hoffman 1990). The specific elements required to estimate crop irrigation water requirement are detailed in Allen et al. (1998). Table 6 below provides an overview of the principal considerations. Table 6 Evapotranspiration Parameters to consider when determining reclaimed water irrigation needs Root zone depth Readily available water capacity Field capacity Leaching fraction Water salinity, SAR and soil salinity The amount of moisture (mm) lost from both the plant (transpiration) and soil (evaporation). Affected by soil and crop type, and environmental conditions (wind, radiation, humidity). A measurement of the depth of soil that the plant can exploit for moisture. The range of soil water contents between field capacity and the point where plants experience yield and/or quality reduction. The maximum amount of water a soil can hold once drainage has stopped. The volume of water above the crop requirement which leaches dissolved salts past the crop root zone, preventing salt accumulation. Where sodium (Na) represents more than 6% of exchangeable cations in the soil, soil sodicity may occur, leading to poor water infiltration and waterlogging, and hard surface crusting soils. High salinity tends to mask the effects of sodicity. (adapted from Kelly et al. 2001) 5

14 In practice the quantities of water applied are usually in the range mm (3 10 ML/ha). In Figure 1 we show the relationship between the amount of water applied and the load of nutrient/substance as a function of the concentration of nutrient/contaminant in the reclaimed water, and crop type. From this it can be seen that loadings per crop grown are typically >1 t/ha each for sodium (Na), chloride (Cl), bicarbonate (HCO 3 ) and sulphate (SO 4 ), ca kg/ha for nitrogen (N), potassium (K) and calcium (Ca), <25 kg/ha for phosphorus (P), <5 kg/ha for iron (Fe) and boron (B), and <0.5 kg/ha for heavy metals. In situations where more than one crop is grown in a year, actual loads/yr would be much higher. Those compounds in the high loading group (Na, Cl, HCO 3 and SO 4 ) contribute substantially to soil salinity, and Na to sodicity (structural decline), and these represent immediate on-site management challenges for reclaimed water irrigation systems. Although N and P represent a positive opportunity for crop nutrient management, they can cause off-site problems. Nitrogen may be a threat to groundwater bodies through nitrate (NO 3 - ) leaching, and P contributes to eutrophication of surface waters if run-off occurs. Boron loads in reclaimed water may result in B toxicity for sensitive crops. While the loads of heavy metals are not likely to cause any immediate problems, they may accumulate in the soil following long term irrigation and become a threat to animals (including humans) through food chain transfer. What is the fate of these compounds carried in reclaimed water in the plant-soil-water system? mg/l 100 mg/l Concentration range in reclaimed water sodium chloride bicarbonate sulphate mg/l elemental load (kg/ha) mg/l 1 mg/l nitrogen potassium calcium phosphorus iron boron mg/l 1-5 mg/l mg/l mm irrigation water applied heavy metals <0.1 mg/l peppers tomatoes cauliflower cabbage broccoli carrots turnip potatoes onions vines citrus Figure 1 Relationship between amount of irrigation water applied (mm), concentration of inorganic compounds (mg/l) and elemental load (kg/ha). The approximate range of concentrations for some reclaimed water constituents are indicated on the right, and the approximate irrigation water needs of a range of crops shown underneath. The figure provides a guide to the relative loadings of nutrients, heavy metals and salts for different types of crops. For example, for a crop of potatoes which would typically receive some mm irrigation water would add more than 1t/ha of salt, <100 kg/ha of nitrogen and <10kg/ha boron. Much larger loadings would be anticipated for a citrus crop and about half for tomatoes. 6

15 The largest fluxes from the soil are as nutrients taken up by crop plants, typically 25 kg/t dry matter for N, 10 kg for K, and >2 kg for P, Ca, Mg and S. The remainder of the elements are only required sparingly by plants and will remain in the soil unless moved in water via runoff or leaching. However, crop uptake does not equate directly to loss from the soil system as only the harvested crop products are exported, the remainder would be returned to the soil as crop residues. Table 7 gives an indication of the uptake of major plant nutrients in crops in kg/ha. From this and Figure 1 it can be seen that crop uptake of N, P and K might often approximate the amounts applied in reclaimed water irrigation and thus these might be less likely to accumulate in soils. For the other components crop uptake will typically be much less than that supplied through reclaimed water irrigation. Reclaimed water usually contains enough zinc to correct soil deficiencies within 1-3 years (Westcot and Ayers 1984), depending on the degree of treatment and soil properties. For nitrogen, much additional N is also likely to become available from mineralisation of soil organic matter, which will be enhanced under irrigated conditions. For example, in an effluent irrigated pine plantation in Australia N mineralisation rates of up to 410 kg N/ka/yr were recorded (Polglase et al. ). An over-fertilisation of N may thus result for some crops irrigated with reclaimed water. Table 7 Approximate nutrient uptake in vegetable crops Uptake (kg/ha) Crop Yield Crop fraction N P K Ca Mg Cabbage 50 t/ha Total Capsicum 20 t/ha Total Carrots 44 t/ha Root Leaf Total Cauliflower 50 t/ha Curd Leaf Total Celery 190 t/ha Total Cucumber 18 t/ha Fruit Leaf & Stem Total Lettuce 50 t/ha Total Potato 40 t/ha Tuber Leaf & Stem Total Tomato 57 t/ha 194 t/ha Leaf & Stem Fruit Total Leaf & Stem Fruit Total (modified from Creswell and Huett 1998) Irrigating with reclaimed water: Impacts on the soil resource "the success of effluent irrigation depends on the ability of the soil to assimilate the water, nutrients, and any other contaminants that are applied to it". (Bond 1998) Assessing the viability (sustainability) of a reclaimed water irrigation system requires an assessment of the capacity of the soil to store contaminants from the irrigation water in such a way that crop production and water resources are not degraded via the mechanisms highlighted in Table 1. Problems which have the potential for rapid development are nitrate leaching, salinity and sodicity (Bond 1998). The other factors are relatively less pressing, but still require attention once these primary issues are addressed. Nitrate leaching is a function of the nutrient content of 7

16 the reclaimed water, crop nitrogen demand (uptake) and the leaching rate. The final result of nitrate leaching may be contamination of drinking waters. In contrast, salinity and sodicity issues are likely to manifest initially through poor crop performance due to unsuitable soil conditions in the rooting zone. What constitutes a suitable soil for irrigation? The main factor affecting the sustainability of a reclaimed water irrigation system is the interaction between the total salts in the soil and the irrigation water, the ratio of sodium to other cations (sodium absorption ratio, SAR), and the hydraulic conductivity of the soil. Together these control salinity, soil structure and leaching. Salinity reduces plant growth through osmotic and toxicity effects, a high SAR causes sodicity (soil dispersion/swelling) which increases soil resistance, reduces root growth, and reduces water movement through the soil due to reduction in hydraulic conductivity (Rengasamy and Olsson 1993). This reduces the opportunity for leaching of salts from the root zone and may cause anoxia through waterlogging. The (saturated) hydraulic conductivity of soils (K sat ) may range from 36 cm/hour for beach sand to «1 for heavy clays (Table 8). For successful irrigation with reclaimed water K sat should ideally be in the range 1-15 cm/h (Brady and Weil 2002). For soils of very low K sat, artificial drainage can be installed to decrease the likelihood of waterlogging. Table 8 K sat (cm/hour) Approximate values for soil saturated hydraulic conductivity and impacts on utility Utility 36 Typical of beach sand 18 Typical of very sandy soil, too rapid to effectively filter pollutants in wastewater 1.8 Typical of moderately permeable soils, K sat between 1 and 15 cm/h considered suitable for most agricultural, recreational and urban uses calling for good drainage 0.18 Typical of fine textured, compacted or poorly structured soils. Too slow for proper operation of septic tank drain fields, most types of irrigation, and many recreational uses such as playgrounds <3.6 x 10-5 Extremely slow; typical of compacted clay. K sat of 10-5 to 10-8 cm/h may be required where Salinity impermeable material is needed, as for wastewater lagoon lining or landfill cover material (from Brady and Weil 2002) Crop response to soil salinity All reclaimed water contains salts and salinity should be one of the first things to consider in the development of a reclaimed water scheme. If the soil salinity is too high (electrical conductivity of a saturated soil extract (EC se ) > 3 ds/m), there is likely to be problems for plant growth - variability in plant tolerance to salinity is discussed below. As plants typically retain <2% of the water they take up, the concentration of salt in the plant will be >50 times that of the soil solution, unless it is excluded at the root. The effect of salinity may be osmotic, impacting on water uptake, or due to specific ion toxicities, particularly Na + or Cl - in leaves where they are left behind from the transpiration stream (hence toxic symptoms often first appear in tips or margins of older leaves). Plants which exhibit salinity tolerance usually have an ability to exclude or control Na + and Cl - uptake (Storey and Walker 1999). Since plants typically have a relatively low requirement for Cl -, generally no more than that provided in rainfall, the amounts presented in reclaimed water will likely accumulate to toxic levels for many crops. Similarly for Na +, in the field situation, deficiency has never been observed (Grattan and Grieve 1999b)even though some 8

17 crops such as the beets (Beta spp.) have a relatively high requirement for Na +, which under normal conditions contributes substantially to osmotic adjustment (Mengal and Kirkby 1978). If sprinkler irrigation is used, direct absorption into leaves may also occur and toxicity symptoms manifest at relatively low irrigation water salinities (Westcot and Ayers 1984). Another way in which salinity may affect crop growth is reportedly through interference with nutrition of other elements (e.g. Kaya 2002). There is voluminous literature on the effects of salinity on crops, since it is a widespread problem relating to many irrigation schemes and to dryland agriculture. There is some confusion in the literature in relation to plant responses to salinity and its amelioration, much of which may be related to the variety of experimental conditions used. For example, Grattan and Grieve (1999b) highlighted that where plants are grown in nutrient deficient conditions at a range of salinities, they may respond to increased nutrition. This gives the impression that improved nutrition can overcome effects of salinity, when in fact the response is to nutrients and there is no interaction with salinity. Also the types of salts used in experiments can influence results. Most controlled experiments utilise NaCl as the only salt, whereas in many irrigation waters Ca 2+ is also an important contributor to salinity, particularly at lower salinity levels. During evaporative concentration, Ca 2+ tends to precipitate out, so the Na + /Ca 2+ ratio would be expected to increase with lagooning of reclaimed waters. Nevertheless the response of plants to Ca ++ and Na + are 2- - different. Similarly, the anion composition is important with SO 4 and HCO 3 making up considerable fractions of the ionic mix in saline waters (Asano et al. 1985), but is most often neglected in pot studies. The actual combination of salts/ions in the soil is important since the ratios of these ions dictate both the chemical and physical properties of the soil and their suitability for crop growth (Rengasamy and Bourne 1998). Furthermore the soil water content at which experiments are done may also influence results (Shannon and Grieve 1999) as the concentration of salt in soil water increases as the soil dries out (Rengasamy 2002). It is not always possible to state categorically at what concentrations of which salts, problems will arise for particular crops - since the varied osmotic and toxicity effects of saline soils on plants depends on specific characteristics of the plants and other soil factors. However, crops can be ranked in general terms to their sensitivity to saline irrigation waters and saline soils. The important points to note are: (a) vegetable crops are generally more sensitive to salinity than cereal crops; (b) many woody fruit crops are very sensitive to salinity, but saline tolerant root stocks for increasing the salinity tolerance are only really available for grapes and citrus crops; (c) sensitivity to salinity increases with soil clay content; and (d) for some species, sensitivity increases with leaf exposure to sprinkler irrigation with saline water. The effects of salinity on horticultural crops were well reviewed in 1999 for vegetable (Shannon and Grieve 1999), citrus (Storey and Walker 1999) and tomato (Cuartero and Fernandez-Munoz 1999) crops, as was the interaction between salinity and mineral nutrition (Grattan and Grieve 1999b). Citrus has been extensively studied, probably because it receives the greatest salt loads (see Figure 1) and is susceptible to chloride (Cl - ) toxicity which causes leaf burn (Maas 1987). The major pathway used to alleviate this is through the use of rootstocks which are able to greatly reduce the uptake of Cl -. The chloride concentration in leaves ranges at least 10 fold between salt excluding and salt sensitive root stocks. In the case of Na + there is little evidence for marked differences in ability to exclude salt at the root and Na + concentration varies by no more than 6 fold. In general, yield of citrus declines by about 13% for each 1 ds/m (saturated soil extract) above approximately 1.7 ds/m in the soil (Table 9; ANZECC and ARMCANZ 2000). For three year old Navel orange trees in Spain, Reboll et al. (2000) found no decline in fruit yield when flood irrigated with reclaimed water for three years. They did not indicate whether the plants were own-rooted or on salt-tolerant root stocks. Soil Cl - levels at their site were relatively low, however, it is likely that in time the salinity level of the soil will increase and this could cause problems in a longer time frame than considered in their study. Clearly, when 9

18 growing citrus trees with saline waters, trees should be grafted on to salt excluding rootstock to minimise yield losses. However, this may have little impact on specific ion toxicity (Na + and Cl - ) if sprinkler irrigation is used and saline water is deposited directly on to the foliage of the scions that are not able to exclude salt. Table 9 10 Average root zone salinity tolerance of vegetable and fruit crops, threshold irrigation water salinities before yield loss as a function of soil type, and % yield loss/ds/m after threshold is reached Maximum irrigation water Average salinity before yield loss root salinity ds/m % Yield tolerance sandy loamy clay loss Common name Scientific name (ECse ds/m) soil soil soil /ds ECse Beet sugar Beta vulgaris Kale Brassica campestris Zucchini Cucurbita pepo melopepo Rosemary Rosmarinus lockwoodii Asparagus Asparagus officinalis Beet, garden Beta vulgaris Olive Olea europaea Peach Prunus persica Squash, scallop Cucurbita pepo melopepo Broccoli Brassica oleracee Cauliflower Brassica oleracea Cucumber Cucumis sativus Pea Pisum sativum L Squash Cucurbita maxima Tomato Lycopersicon esculentum Rockrnelon Cucumis melo Spinach Spinacia oleracea Cabbage Brassica oleracea (var. Capitata) Celery Apium graveolens Grapefruit Citrus paradisi Orange Citrus sinensis Potato Solanum tuberosum Pumpkin Cucurbita pepo pepo 1.7 Sweet corn Zea mays Broad bean Vicia faba Almond Prunus dulcis Grape Vitis S pp Pepper Capsicum annum Plum Prunus domestica Sweet potato Ipomoea batatas Avocado Persea americana

19 Maximum irrigation water Average salinity before yield loss root salinity ds/m % Yield tolerance sandy loamy clay loss Common name Scientific name (ECse ds/m) soil soil soil /ds ECse Lettuce Lactuca sativa Onion Allium cepa Radish Raphanus sativus Eggplant Solanum melongena Apple Malus sylvestris Bean Phaseolus vulgaris Carrot Daucus carota Lemon Citrus limon Pear Pyrus spp Strawberry Fragaria spp Turnip Brassica rapus (se = saturation paste extract) (collated from ANZECC and ARMCANZ 2000; Maas 1987) Grapevines are similar to citrus with respect to the impact of rootstock on salinity tolerance of scions, and thus changing the rootstock can eliminate leaf burn and maintain grape yields (Walker et al. 2002). Cass et al. (1995) highlighted that most vines in Australia would be suffering substantial yield penalties due to salinity of normal irrigation waters and soils if not grafted on to salt tolerant rootstocks. Table 9 collates data, derived from a number of sources, but principally (ANZECC and ARMCANZ 2000; Maas 1987), on the sensitivity of a range of crops to saline irrigation water and saline soils. Cuartero and Fernandez-Munoz (1999) summarised research on salinity and tomatoes. The key points were: (a) Germination and early growth is sensitive to salinity; (b) Salinity tends not to affect the dry matter distribution between fruit, shoot and root; (c) Fruit weight, but not fruit dry matter, declines with increasing salinity, thus the effect of saline irrigation water is probably osmotic rather than a specific ion toxicity; (d) Fruit development and maturation is faster under saline conditions; (e) Blossom end rot, caused by a local Ca ++ deficiency at the distal placental fruit tissue, can increase under saline conditions due to reduced Ca ++ uptake; and (f) Crop nutrition needs to be optimised to minimise effects of salinity wherever possible. Attention should be paid to maintaining high Ca ++ nutrition that can help reduce Na + uptake and increase both Ca ++ and K + uptake, which are generally depressed under saline conditions. Overall, tomatoes provide an attractive option for irrigation with reclaimed water, for although they are considered moderately sensitive to salinity, fruit size (and thus yield) does not decrease until salinities (EC se ) rise above 2 ds/m in most soils. Managing root zone salinity As indicated earlier most of the water that is applied to crops and soils is returned to the atmosphere via evapotranspiration, leaving the salts behind. Obviously the more water applied as irrigation, the more salt is added. Adding up the contributions from Na, Cl, HCO 3 and SO 4 in Figure 1, it can be seen that several tonnes of salts would typically be added with an irrigation of 500 mm of reclaimed water. As plants generally contain no more than 3.5% salt in dry matter (Hoffman 1990) no more than 350 kg of the salt applied in irrigation water would be exported in 10 t/ha of harvestable products. The salt concentration in the soil also increases with each irrigation, unless the salt is leached down the soil profile and beyond the crop rooting zone by an 11

20 excess of water above that which the soil can store (the drained upper limit, DUL) (Oster 1994). In irrigated soils (including reclaimed water) the salinity of the soil at the surface is the same as that of the irrigation water, whereas at the bottom of the profile (below the root zone) it is likely to be much greater, depending on the leaching rate (Oster et al. 1996). For example, with a leaching fraction of 0.1 (10%) the salt concentration at the wetting front will be ten times that of the irrigation water applied. Stevens et al. (2003b) report mean soil salinities from sites under long term (<28 years) irrigation with reclaimed water or bore water, and soils not irrigated or used for agriculture (Figure 2). From this it can be seen that while the salinity of surface soil (0-30 cm) at the end of a cropping cycle was higher under irrigated (EC se approximately 3 ds/m) than unirrigated, non agricultural soils (EC se < 2 ds/m), the salinity of the soil below 40cm was much lower under irrigated (EC se 3.5 ds/m) than unirrigated, non agricultural soils (EC se > 5.5 ds/m). This clearly demonstrates both the deposition and accumulation of salt from irrigation waters, and the leaching of this salt beyond 1m depth. For this study, threshold salinities (EC se 3 ds/m) for effects on crop growth (see Table 9) would be anticipated at 30cm under reclaimed water irrigated soils, 50 cm under bore water irrigation, and 40 cm for the unirrigated soils. They concluded that an adequate leaching fraction must have been used over the years to maintain soil surface (0-30cm) salinity below 3 ds/m. For deep rooted species, such as perennials one would anticipate that the lower salinity irrigated soils provided a better deep rooting environment than the unirrigated soils. Salinity effects begin Soil depth interval (cm) Bore Reclaimed water Virgin Maximum standard error (0.1) (0.2) (0.3) (0.4) (0.7) (1.2) (2.0) EC (ln x 10) (EC) Figure 2 Changes in soil electrical conductivity (EC 1:5 ) with depth related to historical water use. EC 1:5 scale is a natural log transformed because data were not normally distributed. The red line indicates the level that salinity is likely to affect many horticultural crops (from Stevens et al. 2003b) EC se for these data are EC 1:5 x 8.2 (Kelly et al. 2001). The red vertical line indicates the salinity (EC se 3 ds/m) at which crop yield might begin to be depressed. In terms of crop growth, the average EC of the root zone is considered to be the measure of salinity for crop protection (Oster 1994). Where the EC of the root zone is too high for maximal crop production, it can be reduced by the leaching of salts. To displace salt from a volume of soil, the soil could (a) be filled with water to its maximum capacity (DUL) and then (b) this displaced with the same volume of irrigation water. Thus, nominally, twice the volume of the DUL in the crop rooting zone would achieve a salinity equivalent to the irrigation water used (assuming no evaporative concentration). Given the DUL s in Table 10, and assuming a rooting depth of 50cm (Table 11) some mm of water would be required to displace the soil water from 0-50cm down to cm, depending on the soil type. This is a simplification since it does not take into account downward movement of water via unsaturated flow, or capillary 12