Over the past several years, a number of tailwater recovery systems have been constructed on row crop production areas in Yolo County. Irrigated row crop fields generally drain into an intermediate drainage system (e.g., furrows and ditches) that eventually empties into a larger drainage such as a canal or slough. The water coming off these fields during summer irrigation and winter storms is lost from the farm, often taking with it clay, silt, organic matter, and agricultural chemicals.
The tailwater recovery system is designed to catch and store at least part of this runoff water for reapplication or for other beneficial uses. In addition to conserving irrigation water supplies, the tailwater recovery system reduces the amount of suspended soil that is normally lost through runoff. The system may also provide a recharge source for underlying aquifers as well as a valuable habitat for the local fauna and flora. Tailwater recovery systems can be used in areas where recoverable irrigation runoff can be anticipated under the management practices used or expected to be used.
The tailwater recovery system studied in this protocol consists of two adjoining water bodies. The first basin, which receives the initial runoff from fields, acts as a sedimentation trap. Collected sediment can be removed and reapplied onto the fields. The second basin is used for water storage, ground water recharge, and wildlife habitat. Construction of tailwater recovery systems as a management practice is being promoted by the Yolo County Resource Conservation District.
For the purpose of this study, water and the sediment in the sediment basin and recharge pond in this study are considered on-site. Presence of diuron in the soil below the recharge basin or in the water in the recharge basin that is discharged into the slough will be considered as indicators of potential off-site movement of residues. To determine potential off-site movement of residues from this tailwater system, diuron concentrations will be measured in soil sampled from the recharge basin at the end of the growing season and in water periodically sampled from the recharge pond throughout the growing season.
The objective of this study is to obtain a preliminary assessment of the ability of the tailwater recovery system to prevent (mitigate) off-site movement of diuron to either surface or ground water.
This study will be conducted by the Environmental Hazards Assessment Program (EHAP) under the general direction of Don Weaver, Senior Environmental Research Scientist. Key personnel are listed below:
Authors of the final report will include but may not be limited to Blanca Rodriguez, Nan Singhasemanon, John Troiano, Duc Tran, and Terri Barry, . Questions concerning this monitoring program should be directed to Pat Dunn at (916) 324-4100 and FAX (916) 324-4088.
IV. EXPERIMENTAL DESIGN / SAMPLING METHODS
The selected tailwater recovery system receives water from a row crop field that is partitioned into 15 blocks planted with 14 native grasses and wheat. Water is not being recirculated through this system. The tailwater recovery system at this site is intended to trap sediment and to provide ground water recharge and wildlife habitat. Diuron will be applied to the field in the spring to suppress weed growth in the native grass crop. Water will be periodically sampled from the recharge pond to determine potential for movement of diuron residues from the field to surface water. Soil from the recharge pond will be sampled at the end of the season to determine potential for residues that are present in the recharge pond to move downward toward ground water. Additionally, water samples will be obtained from the sediment basin at the occurrence of the first runoff since these samples will likely contain the highest concentration of diuron residues.
Sediment Basin Sampling
The first irrigation event will be monitored to estimate the mass of diuron transported from the field into the sediment basin. Four composite water samples will be taken in the sediment basin prior to the first irrigation. The basin will be divided into five equal sections. One subsample will be taken randomly from each of the five sections. Four more composite water samples will be taken from the sediment basin in the same manner following irrigation.
Each water sample from the sediment basin will be later split into two separate samples: 1) whole water samples for total diuron determinations, and 2) filtered samples for dissolved diuron determinations. The concentration of diuron in the suspended sediment will be calculated as the difference between concentrations in the whole and filtered water samples.
Irrigation Canal Sampling
Two water samples will be taken from the adjacent irrigation supply canal during the first irrigation event. These two samples will be grab samples and will be analyzed as whole water for total diuron.
Recharge Pond Sampling
Water: Before runoff from the treated field occurs, four composite water samples will be taken to determine the background concentration of total diuron in the recharge pond water. Each of these four samples will compose of eight subsamples. The pond will be divided into eight approximately equal sections. One subsample will be taken from each of the eight sections.
Four additional composite water samples will be taken after each runoff event until the end of the irrigation season. There will be a time delay between the runoff event and the recharge pond sampling because the clay and silt will need to adequately settle before representative water samples can be taken from the recharge pond.
Soil: At the end of the irrigation season, four soil cores will be taken in the dry areas at the edge of the recharge pond. Each core will be 36 inches deep and divided into three 12-inch sections. Diuron concentrations in soil cores taken from the recharge pond will be calculated on a dry weight basis.
If water overflows from the recharge pond into the adjacent slough during the study, two grab water samples will be collected from the first overflow event. These samples will be analyzed as whole water only for total diuron.
Additional data collected will include dimensions and depth of the basin and pond area, volume of water applied, physical measurements (e.g., pH, temperature), and on-site rainfall measurements if applicable.
V. CHEMICAL ANALYSIS / QUALITY CONTROL
The California Department of Food and Agriculture laboratory will determine diuron concentrations in whole water, filtered water, and soil. The reporting limits for diuron for the three sampling matrices are to be determined by the CDFA laboratory.
The quality control program will include the following: A solvent blank and a matrix spike will be analyzed with each extraction set. Results of matrix spikes will be compared to the warning and control limits established. The CDFA laboratory will be responsible for filtering all water. The results from the method validation study will be used to establish recovery control limits for the field study.
VI. STATISTICAL ANALYSIS
The average concentration of diuron in the sediment basin, the recharge pond, and in the soil cores at specific points in time will be calculated.
This limited study is designed to address the following questions:
1) Is diuron present in water sampled from the recharge pond and what is the maximum diuron concentration measured?
2) Is diuron present in the soil below this recharge pond at the end of this irrigation season?
Sediment Basin - (first irrigation only) = 16
Irrigation Canal - (first irrigation only) = 2 Whole water
Recharge Pond - (7 sampling events x 4 replicates) = 28
Overflow - (first overflow only) 2 replicates = 2
Total = 60