Investigation into Effect of Soil Moisture Depletion on Vegetable Crop Uptake of Microcontaminants Under Recycled Water Irrigation
U.S. Department of Agriculture
01/01/11 – 12/31/13
The long-term goal of this study was to determine the effects of soil moisture stress on the uptake of microcontaminants of potential human health concern found in wastewater irrigation by food crops generally eaten fresh. A better understanding of these effects can guide agriculturalists in implementing their irrigation scheduling to minimize such uptake. This will also help to resolve emerging concerns about the long term safety of utilizing effluent for the irrigation of crops eaten fresh. Greater latitude in the use of wastewater for crop irrigation will help to strengthen the nation’s food security, particularly in areas with limited irrigation resources.
We know that as populations continue to grow, and rainfall and other climate parameters become more erratic, we will need to reuse an increasing amount of our effluent to grow crops. In its 2007 report, the Intergovernmental Panel on Climate Change (IPCC, 2007) predicts extreme variability in rainfall on a decadal scale on Pacific islands, where more than three million people reside. Up to 25% reductions in rainfall are expected in the next five decades given the current rates of temperature and carbon dioxide rise in the atmosphere. Municipal effluent use, properly managed, is becoming a necessity for many of these islands. Pharmaceuticals and Personal Care Products (PPCPs) used for personal health or cosmetic reasons comprise a diverse collection of thousands of chemicals, including prescription and over-the-counter drugs, perfumes, fragrances, and cosmetics. Considerable quantities of these end up in our cities’ wastewater treatment plants where they are removed or denatured to greater or lesser degree. Many of these chemicals are ubiquitous in wastewater effluent and receiving waters. Some are extremely refractory, making their way even through modern drinking water treatment plants. Advances in technology have improved our ability to detect and quantify these chemicals at extremely low levels, and considerable attention is now being focused on the effects these chemicals in the environment may have on human health. There is concern in the regulatory and scientific community that many of these chemicals (including actual human hormones) may have subtle effects on the human endocrine system when ingested in extremely low concentrations over the long term.
While the effects of most PPCPs are well known at therapeutic doses, the health implications of long-term exposure to these chemicals in the low concentrations found in wastewater, and indeed in many drinking water supplies, were poorly understood. The general consensus had been that the health implications of exposure to the extremely low concentrations of PPCPs in the environment are negligible. A 2005 technical brief on endocrine disrupting compounds and implications for wastewater treatment prepared for the Water Environment Research Foundation states that no studies reviewed as of that date effectively linked low concentrations of endocrine disrupting compounds (EDCs) to adverse health effects in human. Nonetheless concerns are growing over the constant, ubiquitous exposure to the complex mixture of these chemicals that we are increasingly subject to in the environment, our drinking water, and our food. Although negative health effects of PPCPs at extremely low levels such as those found in drinking water (and most probably in crops irrigated with effluent), have not been documented, it is only prudent to ensure that agricultural practices do not result in higher levels making their way into the food supply.
The human endocrine system is an exquisitely-balanced, finely-regulated network of chemical communication. Infinitesimal adjustments occur constantly to regulate body functions. If this system is disturbed the consequences can be dramatic and dire. Disruptions of the endocrine system are associated with a host of human diseases such as diabetes, cancer, and hypertension, to name but a few.
Potentially-endocrine-disrupting chemicals are abundant in wastewater. In addition to the actual human hormones excreted by populations there are many PPCPs with the potential to disrupt endocrine functioning. Given the extreme potency of hormones’ effects on human health it is feasible that long-term exposure to even very low doses of hormones and chemicals that interfere with hormonal functioning will cause health effects. While negative health effects have yet to be conclusively demonstrated from this exposure, the possibility is one that is increasingly of concern to environmental scientists and epidemiologists (National Toxicology Program’s Report of the Endocrine Disruptors Low-Dose Peer Review, August 2001).
Another concern related to using effluent for irrigation is that low levels of antibiotics introduced into the soil through wastewater irrigation may lead to the evolution of resistant bacterial strains. The phenomenon of bacteria developing resistance to antibiotics through the overuse of these chemicals is well known to public health practitioners. The constant, low-dose exposure of whole populations to these drugs by the route of drinking water and food may be resulting in the evolution of pathogenic bacteria against which we will have no defenses. From the agricultural perspective the application of antibiotics to soil may cause harmful changes to the community of bacteria living there and in the rhizosphere.
According to researchers looking at changes in antibiotic resistance in soil bacteria in samples taken and archived in the Netherlands between 1940 (when antibiotic use was beginning to be widespread) until 2008, “…there is growing evidence that resistance to antibiotics is increasing both in benign and pathogenic bacteria, posing an emerging threat to public and environmental health in the future.” (Charles Knapp et al., Environ. Sci. Technol. 2010, 44, 580–587)
One 2003 study tested 64 soil bacteria isolates collected from fields that had been irrigated with effluent for at least a decade near Aligarh, India for antibiotic resistance. The isolates were tested for their antibiotic susceptibility against seven different antibiotics, nalidixic acid, cloxacillin, chloramphenicol, tetracycline, amoxycillin, methicillin, doxycycline. The researchers reported that 100% of the Pseudomonas isolates were resistant to cloxacillin and 57.5% were resistant to methicillin. 7.5% of the isolates exhibited multiple resistance to five different antibiotics in three different combinations whereas 25% of the isolates showed multiple resistance to four different antibiotics in seven different combinations.
While the scientific and regulatory community has mostly turned its attention to the issue of PPCP contamination of drinking water, another potential route of exposure to wastewater PPCPs has received little attention. That route is via crop irrigation with effluent. As populations grow and water supplies dwindle in many areas, agriculture is turning to treated municipal effluent as a source of irrigation water. The practice is common worldwide.
ALAR facility in Maricopa
A wide range of factors determine the rate of water and chemical uptake by crops (plant species, root structure, soil surface charge, other soil characteristics, transpiration rate, air temperature, humidity, nature of the chemical, concentration of the chemical, soil moisture depletion, etc.). The uptake of necessary nutrients by crops under different conditions of water stress has been well characterized however the same cannot be said of other types of chemicals. Some plants are known to bioaccumulate potentially toxic chemicals and in fact are employed in spill-site cleanup because of this capability. Studies have demonstrated that increased transpiration results in enhanced translocation of metals by plants used for intentional phytoremediation of contaminated soils. Transpiration rates are directly affected by the availability of moisture to the plants’ roots. So in addition to differences in concentrating capacity attributable to the type of species it is logical to assume that this capacity is also a function of growing conditions generally, and of soil moisture depletion/water stress specifically. The rate and magnitude of chemical uptake by crops may thus be at least partially affected by the irrigation regimen under which they are grown. It is this issue that we are addressing with our study.
Rationale and Significance:
One of the stated area priorities of the USDA Agriculture and Food Research Initiative (AFRI) Program – the project funding agency – is to understand the potential and relevance for bioaccumulation of recycled water constituents applied at typical irrigation rates used exclusively or through blending with surface and groundwater sources. We examined the bioaccumulation of PPCPs by crops under irrigation with water containing these chemicals in concentrations typically found in recycled water. We wanted to help address the concerns over potential health risks posed by consuming raw food crops that may bioaccumulate these chemicals. Information generated by this study has helped characterize the uptake of PPCPs under different soil-moisture stress conditions and have applicability for designing irrigation regimens for food crop production. Being able to greater utilize this source of irrigation water will permit farmers to put into production more acreage in areas where irrigation water availability is a limiting factor. There is a need for studies that examine PPCP uptake by food crops particularly those which are eaten raw. This was the gap in the knowledge that we were addressing with this project.
We determed the rate and magnitude of uptake from irrigation water spiked with varying concentrations of selected PPCPs usually found in municipal effluent by several different vegetable crops. We made these determinations by applying concentrations of chemicals normally found in wastewater and multiples thereof under various irrigation regimens, permitting varying degrees of soil moisture depletion. By holding other variables affecting the uptake of chemicals by crops consistent, we characterized the importance of: 1) chemical concentration, and 2) soil moisture depletion/water stress, in this uptake process. The studies were performed in greenhouses in order to control external variables to the largest degree possible.
The studies were conducted near Maricopa, AZ and in Honolulu, HI. Both of these locations have restricted water resources and there was considerable interest on the part of local governments in reusing wastewater for irrigation. In Hawaii, the goal of reducing the discharge of municipal effluent into nearshore waters was an added impetus to expand the limited use that is currently being made of wastewater effluent.
Each of the greenhouse studies employed five vegetable crop types: tomatoes, green onions, cucumber, carrots, and lettuce. The plants were each replicated three times in multiple beds, over multiple trials, for each possible combination of soil moisture depletion and PPCP concentration (typical wastewater, 2x, 5x, and 10x) in initially sterile inert media. We used three compounds normally found in wastewater, one antibiotic, one prescription drug, and one over the counter drug. In addition, we used caffeine as a general marker for wastewater as it is found in higher concentrations.
Irrigation was with non-chlorinated potable water from the municipal system in Hawaii, and from a groundwater source uninfluenced by treated wastewater in Arizona. For each of the experimental crops, this water was spiked with one of the test PPCPs at concentrations similar to those found in municipal effluent and in multiple concentrations thereof. Soil moisture content of the growth media was monitored using time domain reflectometry (TDR) probes and time domain transmissometry (TDT) which controlled an automatic irrigation system. This system delivered identical amounts of water to the beds in response to preset limits of soil water tension (dryness). The irrigation treatments were all based on crop needs but with different frequencies. Irrigation was triggered when the soil moisture reaches certain values and then irrigation water was applied until the same amount of water was applied to each bed. We irrigated using a controlled drip system so that we could quantify the amount of water applied. These treatments allowed for a difference in the time-dependent concentration of the test chemicals in the irrigation water in the vicinity of the roots due to evaporation.
Environmental monitoring included air and soil temperatures, humidity in the greenhouse, soil moisture, soil salinity, and light. Soil temperature, moisture status and salinity were measured and logged every 15 minutes using off-the-shelf buried sensors attached to irrigation controllers that automatically turned the irrigation water on at preset soil moisture potentials and off at saturation. Air temperatures and humidity were also measured and logged every half hour. Light levels were measured and logged every ten minutes.
When the crops reach maturity, the edible parts were harvested and analyzed for the PPCP that was applied to them. Crops were harvested and weighed to determine total yield. Up to five random samples were taken from each of three plants and homogenized according to plant. These samples were analyzed for the compounds using LC/MS. If the parent was not found, this indicated that either a metabolite was taken up by the plant or that the compound was broken down within the plant. Identifying any metabolites was beyond the scope of the proposed study. LC/MS work was done at both the Arizona and Hawaii facilities.
The results of these experiments were disseminated to agriculturalists, agencies, wastewater treatment plant operators and other stakeholders so that they can make better decisions about irrigation with wastewater. Ultimately, this can lead to increased use of wastewater for irrigation, greater resulting crop yields, and reduction of the potential health risks to consumers.
Dr. Chittaranjan Ray, WRRC
Clinton Williams, USDA – Arid-Land Agricultural Research Center
Philip Moravcik, WRRC