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Chapter 11

System Design for Evaluation and Control of Agrochemical Movement in Soils Above Shallow Water Tables

Downloaded by PENNSYLVANIA STATE UNIV on August 6, 2012 | http://pubs.acs.org Publication Date: June 20, 1991 | doi: 10.1021/bk-1991-0465.ch011

System Design for Water Table Management Guye H. Willis, James L. Fouss, James S. Rogers, Cade E. Carter, and Lloyd M. Southwick Soil and Water Research Unit, Agricultural Research Service, U.S. Department of Agriculture, P.O. Box 25071, Baton Rouge, LA 70894-5071 The rate of pesticide transport through soils may be significantly affected by various soil-water/watertable management methods. Bordered plots (16 each, 35 by 61 m, each surrounded by a subsurface 2-m vertical plastic film barrier) will be equipped with 102-mm diameter slotted plastic tubing 1.0 m below the soil surface and with appropriate sumps and pumps for microprocessor controlled subdrainage/subirrigation. Each plot will also be equipped with watertable measuring pipes with depth sensors, soil moisture (matric potential) sensors, soil -water pressure sensors, tensiometer-pressure transducers with ceramic cups, watertable sampling tubes, piezometers, and soil temperature sensors (current generating diode type), all placed at various depths in the root/vadose/ watertable zones and at appropriate distances from the center drainline in each plot. Initial treatments will include four replications each of (i) surface drainage only, (ii) conventional subsurface drainage at a 1-m depth, (iii) controlled watertable at 45 ± 5 cm depth and (iv) controlled watertable at 75 + 5 cm depth. Pesticide and other organic chemical contamination of groundwater has become a national concern t h a t needs timely and r a t i o n a l solution. There are major economic reasons for the continued use of pesticides for the foreseeable future in U.S. agriculture, and there is potential for groundwater contamination from continued, long-term pesticide use. Since groundwater provides drinking water f o r about half of the U.S. population (2), prudence suggests that steps be taken t o r e c t i f y t h i s p o t e n t i a l problem. About 25%, Le., 40 million hectares, of the t o t a l U.S. cropland needs drainage (2). Much of t h i s land i s u s u a l l y f l a t , h i g h l y fertile, and has no serious erosion problems. These potentially productive wet soils are primarily located in the p r a i r i e and level uplands of the Midwest, the bottom lands of the Mississippi Valley, the bottom lands in the Piedmont and h i l l areas of the South, the coastal plains of the East and South, and irrigated areas of the This chapter not subject to U.S. copyright Published 1991 American Chemical Society

In Groundwater Residue Sampling Design; Nash, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.


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West (3). During most o r p a r t of t h e year these s o i l s have shallow watertables that are potential sinks f o r pesticides that may leach below t h e root zone. Pesticides and fertilizers are used extensively in the lower Mis sissippi River valley (LMV), the agriculturally important Mississippi River flood plain in Arkansas, Mississippi, and Louisiana (Fig. 1). Although large quantities of water flow down the Mississippi River, most fresh water supplies for domestic and agricultural use come from the Mississippi River alluvial aquifer (MRA) which underlies t h e LMV. In south Louisiana most water supplies come from shallow wells and surface waters. Pesticide contamination of groundwaters in the LMV has been reported (4 - 8 ). The Mississippi River alluvium i s generally less than 70 m thick and grades downward from silt and clay at the surface to coarse sand and gravel at the base (θ-10). The thickness of t h e o v e r l y i n g silt and clay i s generally less than 12 m. Rainfall, ranging from 1150 to 1500 mm annually, i s the major source of recharge for the aquifer (8,11). The amount of recharge depends not only on t h e amount and rate of precipitation, but also on the permeability and thickness of the overlying s i l t and clay. These deposits are relatively permeable compared to typical clay because of their high content of organic material and because they have not been fully consolidated by heavy overburden (8). Water levels i n t h e MRA generally are less than 9 m below land surface, and are much closer t o t h e s u r f a c e (0 t o 2 m) i n southern areas (8,11). These shallow watertables fluctuate considerably and respond mainly t o rainfall. Conditions a r e present i n t h e LMV f o r s u r f a c e water and groundwater pollution including (a) shallow watertables, (b) high p e s t i c i d e use, and (c) high r a i n f a l l . Concepts of Watertable Management The "optimal" management of soil-water for agricultural cropland i n humid areas of the U.S. via control of watertable depth in the s o i l profile involves complex daily operational/management d e c i s i o n s because of the erratic spatial and temporal distribution of rainfall. The farm management decisions a r e even more complex because soil-water management must be integrated with improved fertilizer and pesticide application practices. Integrated methodology i s needed t o manage soil, water, ground cover, p e s t i c i d e a p p l i c a t i o n s , and fertilizer applications in such a way that pesticides and f e r t i l i z e r s are contained within their "action zones", thus reducing the risk of surface and groundwater pollution. Improved soil-water management technology, e.g., watertable control, may reduce t h e amount of pesticides and f e r t i l i z e r used, thus increasing crop production efficiency and farmer p r o f i t a b i l i t y , while reducing p o l l u t i o n . Periods of excess and deficit soil-water conditions in the active root-zone often occur within t h e same growing season. Thus, controlling watertable depth within a desired range relative t o the root-zone requires facilities for regulating both subsurface drainage flow from and i r r i g a t i o n i n t o t h e s o i l p r o f i l e . A popular field-scale watertable management system uses a subsurface draintube system for both œntrcûled-drainage and subirrigation. Controlled-



11. WILUS ET AL.

Agrochemical Movement in Shallow-Water-Table SoUs

drainage permits retention and temporary storage of i n f i l t r a t e d r a i n f a l l i n the s o i l profile at an elevation above t h e d r a i n l i n e depth. Conventional, or "free", subsurface drainage t o t h e f u l l depth of the draintubes i s often needed during periods of extended or heavy rainfall to control the watertable rise and reduce the duration of excess water in the active root-zone. During subirrigation, the water level at the drain outlet is maintained at an elevated position by pumping from an external source (e.g., a well) to force water back through the draintube system and into the soil profile to manage the watertable at the desired elevation above the drain outlet for proper p l a n t growth. The primary purposes of watertable control are t o minimize the time of excess or deficit soil-water conditions in the root-zone, and to maximize the utilization of natural rainfall, thus minimizing the amount of subirrigation water r e q u i r e d from e x t e r n a l sources. Watertable management technology has also begun to be used to improve water quality. Gilliam and associates developed controlled drainage practices for reducing nitrogen and phosphorus levels in surf ace/subs u r f a c e e f f l u e n t from a g r i c u l t u r a l lands (12-14). These practices are being used i n eastern and southern coastal p l a i n s soils. Watertable management has a high potential for achieving maximum crop production, water use efficiency, and improved water quality i f properly controlled to compensate f o r changes i n weather conditions. Determining when changes are needed i n controlleddrainage and subirrigation to manage the watertable depth optimally is a major problem for farmers, especially in coastal areas with fine textured soils. In the Mississippi Delta frequent r a i n f a l l events can cause large variations in watertable depth because of the small, 3 to 8%, drainable s o i l porosity. Rainfall probability information included i n daily forecasts issued by the U.S. National Weather Service can be used to aid the farmer in making management decisions i n a n t i c i p a t i o n of p r e d i c t e d weather changes (15,16). For level and low-lying topography where subsurface drainage by gravity flow outlets i s not feasible, a sump-type structure can be used for controlling the water level at the drainage system outlet (Fig. 2). Water i s pumped out of the sumps for subsurface drainage and into the sumps for subirrigation. The controlled-subirrigation mode of watertable management i s i l l u s t r a t e d i n F i g . 3. The monitored watertable depth (WTD) midway between the subsurface conduits i s the controlling performance parameter. For conventional subsurface drainage the water level i n the sump i s maintained (by pumping) below the drainline depth. Where gravity flow drainage outlets are feasible, a float-activated control valve on the outlet pipe can be used t o r e g u l a t e drainage e f f l u e n t (17). Research O b j e c t i v e s The r e s e a r c h o b j e c t i v e s of t h i s study a r e t o : 1. Identify and characterize chemical and p h y s i c a l f a c t o r s and processes that affect the rate and mode of pesticide and plant nutrient transport in surface runoff and i n the root, vadose, and saturated zones of shallow watertable s o i l s . 2. Characterize and quantify the effects of water-soluble organic matter on p e s t i c i d e t r a n s p o r t i n s o i l .


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Sump-Controlled

Controlled-Drainage

Watertable Management System Figure 2. Schematic drawing of sump-controlled watertable management system i n the controlled-drainage mode of operation.


11.

WILUSETAL

Agrochemical Movement in Shallow-Water-Table Soik

3.

Determine the effects of watertable management on losses of pesticides and plant nutrients v i a surface runoff, subsurface drainage outflow, and l e a c h i n g t o groundwater. 4. Develop models needed t o d e v i s e watertable management strategies t h a t w i l l avoid and/or a l l e v i a t e groundwater contamination by p e s t i c i d e s and f e r t i l i z e r s .


General Plot Layout and S i t e C h a r a c t e r i z a t i o n The study will be conducted on 16 bordered, 0.21-ha plots (located on the Louisiana Agricultural Experiment Station's Ben Hur farm near Baton Rouge, LA) instrumented for automatic, microprocessorcontrolled measurement and sampling of s u r f a c e r u n o f f and subsurface drain outflow, and watertable management (Fig. 4). The cropping system will be conventionaHy-tilLed corn with common rye grass as a winter cover crop. Previous research (18) has shown that 0.21-ha plots are large enough to minimize "border effects". The plots are on a Commerce s i l t loam s o i l ( f i n e - s i l t y , mixed, nonacid, thermic, Aerie Fluvaquents) (19), which c o n s i s t s of layers of s i l t and clay mingled with sand l e n s e s t h a t were deposited by past Mississippi River overflows. The top 45 cm of the s o i l p r o f i l e i s r e l a t i v e l y high i n clay (= < 34%). Consequently, the hydraulic conductivity i s relatively low and the soil i s easily compacted by wheel t r a f f i c (20). At depths from 45 to 90 cm the clay content decreases to about 22% while the s i l t and sand contents range up to =

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