Economical Monitoring Procedure for Assessing ... - ACS Publications

Chapter 15

Economical Monitoring Procedure for Assessing Agrochemical Nonpoint Source Loading in Unconsolidated Aquifers 1,2

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Roy F. Spalding , Mary E. Exner , and Mark E. Burbach

Downloaded by TUFTS UNIV on June 5, 2018 | https://pubs.acs.org Publication Date: June 20, 1991 | doi: 10.1021/bk-1991-0465.ch015

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Water Center and 2Department of Agronomy, Institute of Agriculture and Natural Resources, University of Nebraska, Lincoln, NE 68583-0844 3Conservation and Survey Division, Institute of Agriculture and Natural Resources, University of Nebraska, Lincoln, NE 68588-0517 Multilevel samplers (MLSs) consisting of piezometers and tube samplers are a logical approach for determining the direction of groundwater flow and chemistry in shallow (< 6 m) nonpoint source (NPS) groundwater investigations. These MLSs have evolved from fastening the tubing to conduit at specific depths while the conduit was lowered into the hollow stem auger train to the present method of installing pre -assembled MLSs in boreholes drilled by thereversecirculation rotary method without the use of drilling additives. This method allows the aquifer to be sectioned into discrete layers and provides an instantaneous snapshot of both flow and chemistry in three dimensions. The procedure has been used successfully at several sites in Nebraska. The method is cheap, fast, and accurate in areas where the depth to water is less than 6 m. While the same procedure can be used where depths to water exceed 6 m, the need for gas-driven samplers substantially increases the cost.

The problems with setting criteria for devising groundwater sampling strategies probably are best summarized by the statement that no two field sites or basins are identical. In spite of that fact, investigators still must follow logical protocols to attain their goals; otherwise, the comparative framework of theirfindingswill be sacrificed. Guidelines for techniques, however, must be flexible if they are to be effective in a variety of scenarios. While past sampling emphasis has been primarily on characterization of point source-contaminated sites (Superfund activities), the focus of the 1990s is rural America and its nonpoint source (NPS) agricultural problems. During the height of the Superfund boom in the 1980s many effective and accurate sampling techniques were developed. It is important that the knowledge gained during that decade serve as the basis for innovative approaches characterizing NPS problem areas. Monitoring NPS contamination, however,requiresspecial sampling designs because the contamination resultsfromvery large input zonesrepresentingwhole basins. Consequendy, it lacks discrete centroids of high concentrations, obvious source areas, and high density loading effects. While extensive sampling installations are not critical in ambient monitoring networks used to delineate the boundaries of large areas of NPS contamination, 0097-6156/91/0465-0255$06.00/0 © 1991 American Chemical Society

Nash and Leslie; Groundwater Residue Sampling Design ACS Symposium Series; American Chemical Society: Washington, DC, 1991.


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GROUNDWATER RESIDUE SAMPLING DESIGN

they are an absolute necessity for defining the effects of prevention technology on groundwater contamination beneath field-size research areas. The effects of improved agricultural management practices on groundwater quality no longer can be inferred solely from input data. Instead their impact on aquifer loading must be documented accurately and precisely by scientific measurements. The need for guidelines for sampling NPS-contaminated groundwater already has been recognized by investigators affiliated with several of the Management Site Evaluation Areas (MSEAs). Several of these research sites were selected to demonstrate that the use of prevention strategies (Best Management Practices) do affect agrochemical loading to the aquifer. In March 1990 thefirstMSEAs were selected in com and soybean production areas of the five states represented in the north-central region of the United States where there is documented or suspected NPS agrochemical contamination. These initial MSEAs are federally funded through the United States Department of Agriculture (USDA), the Agricultural Research Service (ARS), and the Cooperative State Research Service (CSRS) and perhaps in the future by the United States Geological Survey (USGS). Other sites will be added in succeeding years. Many of these sites are in areas where the water table isrelativelyshallow; and in thefinalanalysis, impacts of BMPs will be based on observations of the groundwater. Because data from these sites will be compared, it must be collected in a comparative manner, consequently, sampling guidelines are crucial. Research Site NPS Sampling Methodology Guidelines for all groundwater sampling programs should include pre-installation recommendations for siting monitoring equipment, drilling and logging boreholes, and constructing samplers, and post-installationrecommendationsfor purging samplers and collecting samples. The recommendations presented here integrate methods previously reported for point source characterization with procedural modifications necessary to intensively monitor the fate and transformation of agrochemicals in NPS-contaminated groundwater systems. Theresearchgoals and subsurface environment usually dictate the spatial distribution for sampling design. The direction of groundwater flow can be delineated by triangulation (7) with existing surveyed wells or with at least three piezometers. The piezometers may be driven or jetted sand points with minimum construction standards and used only to delineate the direction of lateral flow or they can be installed to meet higher performance standards and used as part of the site monitoring network. After determining the flow direction by triangulation, an array of multilevel samplers (MLSs) is installed. Instrument locations will vary from site to site and many can best be determined by phasing them in as information is gained from drilling, logging, sampling, and data interpretation. Experience has demonstrated that a step-by-step, phased approach of sampler installation is a wise allocation of time for making informed siting decisions. The final density of sampling sites isrelatedto the lateral statistical variability of agrochemical measurements within discrete vertical layers. This variability may be due to complexities in unsaturated and saturated flow, upgradient agrochemical loading, prior on-site contamination, drilling access and safety, and economics. The importance of the driller's experience with the drilling method, monitoring well construction, and sampler installation cannot be overstated. A contract driller who isreceptiveto delays caused by geologic sampling and logging and is attune to sampler installation protocol should be paramount There are a multitude of methods for installing sampling equipment including mud or air rotary, hammer drive,reversecirculation rotary, cable tool, jet drilling, and solid and hollow stem augering. Nested wells initially were used in NPS investigations (25). The materials and installation, however, are expensive and the wells sample a rela-



15. SPALDING ET AL.

Agrochemical Nonpoint Source Loading

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tively large vertical interval. There is also the potential for cross-contamination because the wells generally are sampled with the same pump. Driscoll (4) and the United States Environmental Protection Agency (USEPA) (56) list the advantages and disadvantages of methods used in well drilling and sampler installation but they do not mention multilevel sampler (MLS) installation which is quickly evolving as the preferred sampling method in most NPS investigations. Experience in the installation of MLSs in shallow unconsolidated aquifers, which are the most vulnerable to contornination, has shown that reverse circulation rotary drilling is a superior installation method The benefits include (1) the absence of drilling additives (bentonite and organics) which tend to invade geologic formations and retard both flow and solute transport, (2) the ability to stop at will during drilling to collect geologic samples, (3) the ability to hold the borehole open during geophysical logging, (4) the ability to maintain large diameter boreholes (18 cm to 1 m) for elaborate system installation, and, most importantly, (5) the ability to seal between samplers where needed Limitations of this method are (1) it relies on large quantities of water which must be free of all analytes of interest and (2) it is not appropriate for drilling in consolidated rock. In the latter situation air rotary or cable tool drilling can be substituted. Water should be analyzed for agrochemical residues and be free of analytes of interest prior to introduction into the borehole. Although MLS installation with a hollow stem auger is relatively cheap, the method has several disadvantages (7). It is difficult to seal between layers and/or samplers; clays and silts smear against the borehole wall; and during retrieval of the auger flight, the auger tends to catch the MLS bundle, kinking the tubing and exhuming the samplers. A variety of monitoring devices in a variety of materials are marketed for groundwater sampling. More conventional sampling techniques such as submersible pumps, auger screen samplers, packer pumps, and regular and Kemmerer bailers used in boreholes and existing wells tend to collect vertically composited samples rather than samples representative of thin discrete vertical intervals. The MLSs have very low pumping rates that do not significantly alter groundwater flow. The ability to economically obtain representative groundwater samples from thin vertical intervals falls almost exclusively to point samplers (MLSs). Since the introduction of MLSs (8-10), several modifications have been introduced. They include gas-driven MLSs for sampling moderate to deep groundwater (77-72) and modular dialysis MLSs (75). Several sophisticated multilevel systems for borehole investigations in bedrock and unconsolidated media are available commercially (14). For most research site NPS agrochemical sampling in shallow groundwater, the less complicated the device, the better. A most appropriate MLS is a combination of tube samplers and piezometers that can be fabricated in the field in up to 30-m lengths. The tubes can discretely sample as many vertical intervals as are necessary to assess loading while the piezometers are used for manually monitoring the water-levels (Figure 1). The discrete samplers are composed of 9.52 mm (3/8-inch O.D.) high density polyethylene (HDPE), stainless steel (SS), or Teflon (PTFE) tubing with screened ports. Screens of SS are purchased locally, cut, and held in place with PTFE ferrules. The samplers are fastened at the appropriate depth to a piezometer that extends to the bottom of the borehole. The piezometer is constructed of 2.54-cm (1-inch) Schedule 40 PVC with a 61-cm (2-ft) slotted interval and capped at the bottom. Additional piezometers are fastened to this assemblage at the appropriate depth. The whole assemblage is put together at the site and lowered into the borehole as a continuous string. Construction materials for the discrete samplers can be of HDPE, SS, PTFE, or some combination of the three. HDPE is much cheaper than SS or PTFE and is probably suitable for sampling chemicals with low sorptivities such as NO3-N, atrazine (2-chloro-4-[etJiylairiinoH^ and alachlor (2-chloro2\6-a4ethyl-N-[methoxymethyl]acetanilide). The USEPA (5) suggests that plastic ,


258

GROUNDWATER RESIDUE SAMPLING DESIGN

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2 m concrete seal ^ water table - 2 0 cm borehole

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6mclean sand or gravel *j

(appropriate size)

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Figure 1. Multilevel sampler installation consisting of piezometers and tube samplers.



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259

casings be checked for sorption by installing adjacent PTFE casings. The conclusiveness of this procedure in point source-contaminated areas, however, is being debated due to aquifer and solute transport variability. With MLSs, however, sorption can be checked by installing all three materials at the same sampling intervals within the same borehole and statistically evaluating the differences. Potential losses from sorption onto the tubing of the peristaltic pump also should be checked. Such experiments for the agrochemicals present in the groundwater beneath the Nebraska MSEA are planned for fall 1990. The total time involved in drilling a borehole with a 21-m completion depth by the reverse circulation rotary method and installing a MLS with eight discrete samplers and four piezometers is approximately 2 h. This includes placing afilterpack of clean gravel or sand in the annular space around the eight discrete samplers and where necessary sealing between sampling intervals with at least a 30-cm bentonite lens. It is important to use precleanedfilterpacks that closely approximate aquifer hydraulic conductivities in order to prevent the introduction of contaminationfromchemical solution and desorption off thefilterpack to the borehole and to minimize disruption of the natural flow system. The location and thickness of the bentonite seals are determined by the discrete sampling intervals, by the formation geology as interpreted from geologic and geophysical logs, and by potential vertical flow components. The seals are formed by chunk bentonite which is dropped slowly into the borehole until the desired thickness, as measured by displacement with a weighted line, is attained. Our experience has been that large chunk bentonite settles faster with less hydration than pellet bentonite. Immediately after installation, the samplers are developed by pumping with peristaltic pumps in shallow (< 6 m) water-table areas or with gas-driven pumps in deeper applications. Development by pumping is an acceptable and well-documented technique (1). As many as 10 samplers can be pumped simultaneously with a multi-module peristaltic pump. A manifold attached to the gas-driven samplers also permits them to be pumped simultaneously. In sand and gravel aquifers sediment-free groundwater is produced quickly (< 20 min) when the reverse circulation rotary drilling technique is used. The same pumps are used for sampling. They are appropriate for sampling low volatile agrochemicals such as NO3-N, atrazine, and alachlor, however, neither pump is recommended for sampling volatiles or gases. Last year at a site near Grand Island, Nebraska, the total cost of a 20-m MLS with eight HDPE discrete samplers and four piezometers was less than $500 and included drilling, MLS materials, and installation. In areas where greater depths to groundwater (> 6 m) require gas-driven dedicated samplers, a similar MLS installation (eight gasdriven samplers and four piezometers) can be purchased and installed for about $4500. At a sludge injection site in Nebraska where a strong NO3-N concentration gradient exists (Spalding, R. F., University of Nebraska-Lincoln, 1989 contract report), the average differences in concentrations between four discrete samplers and the corresponding piezometers in eight MLSs were larger in the shallowest pair and decreased with depth. These average differences indicate that the tube samplers have a more discrete sampling capability than do the piezometers. This capability is important in discerning differences in concentration in the loading zone. The main advantages of MLSs, however, are lowered costs, reduced potential for cross-contamination, and shorter installation time. MLSs have been used in two NPS investigations in Nebraska and the discrete samplers provided definitive results at both sites. A large MLS installation delineated a plume of nitrate contamination (Figure 2) whose source was sludge injected on an irrigated cornfield. MLSs were used near Oshkosh, Nebraska to document agrochemical contamination downgradient from irrigated cornfields (Exner, M . E., University of Nebraska-Lincoln, 1990 contract report).



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Economical Monitoring Procedure for Assessing ... - ACS Publications

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