Nonpoint sampling

The topography of an area must also be taken into account when gridding it for nonpoint sampling [4,5], The tops of slopes will have more erosion of contaminates and soil. Thus, they will generally be lower in contaminant concentration. Lower areas, particularly where water ponds or accumulates, will have higher concentrations of contaminants. Soil color is a valuable characteristic to observe and record in these situations. The tops of slopes will be lighter and typically redder in color, while lower areas will have darker soil colors. 

Millions of soil samples per year are analyzed for nonpoint source analytes such as potassium, phosphorous, insecticides, and herbicides. It is necessary to obtain an idea of the level of these components throughout the field and to take corrective action as necessary. Thus, a more random sampling methodology is called for. Figure 7.2 shows a field that includes both different soil types (letters) and topographical features (e.g., a ditch) that are to be sampled. Also, a transect line and locations of sample sites along it are indicated. Transect lines and sampling sites are discussed later. 

Nonpoint source sampling occurs where the analyte of interest is dispersed over a large area such that a specific point of origin cannot be ascertained. The innate occurrence of analytes of interest would be an example of a nonpoint source. The occurrence of plant nutrients, either naturally occurring or from fertilization, is an example of a nonpoint source of agricultural analytes. Herbicides, insecticides, and pest-control agents are, once applied on a field scale, also potential nonpoint sources of analytes. It is common to think of crop... 

Figure 7.7. Transect sampling for a nonpoint source analyte. 

Transect sampling for nonpoint sources is done the same way and with the same purpose as it is in point source sampling. Figure 7.7 shows a transect across an area to be sampled. Different markers indicate that different sampling will be accomplished at these sites. These would also be sites for depth sampling if it is called for. 

Describe the differences in sampling procedures that would be used when sampling point source and nonpoint source contaminations. Are there any methods of analysis that would be common to both situations ... 

In a groundwater contamination study of 28 of California s 58 counties that evaluated over 50 pesticides (from both point and nonpoint sources), diazinon was detected in 12 samples (Cohen 1986). Diazinon is included as an analyte of interest in the EPA Pesticides in Ground Water Database (EPA 1989a) and was... 

Deethylatrazine (DEA) and deisopropylatrazine (DIA) also have been detected in shallow, unsaturated surface-water runoff from a Eudora silt loam soil with DEA present at higher concentrations (Mills and Thurman, 1994a). Dissolved atrazine, DEA, and DIA concentrations in water samples from two closely spaced lakes indicated large differences in input from watershed nonpoint sources. Levels of these chemicals increased in response to spring and early summer runoff events (Spalding et al., 1994). In studies conducted by Gaynor el al. (1992, 1995), DEA was found in surface runoff samples that contained atrazine. Hydroxyatrazine (HA), deethyl hydroxyatrazine (DEHA), and deisopropyl hydroxyatrazine (DIHA) have also been identified in surface water (Lerch et al., 1995). 

Sources of contaminants could be point source (punctured barrel or tank or pipeline), a linear source (e.g., diesel in a utilities trench or a contaminated recharging stream), a disturbed source (e.g., pesticides and herbicides), a surface source, and an underground source (leaking gasoline tank). Sources of contamination (point source vs. nonpoint source) are scale dependent, thus influencing the sampling plan that will be used. 

The more difficult category of discharge source to regulate is the nonpoint source. This source of discharge covers a wide and varying area and a sample cannot be taken that is truly representative of a specific location. Thus, there are numerous variables that could impact the discharge sample that limits the enforcement of a set of discharge standards for this type of source. 

To determine the influence of sampling frequency on the reliability of water quality estimates in small streams, a cultivated (0.12 km ) and a forested basins (0.07 km ) were studied in spring and autumn [28]. During the 2-month spring season and 3-month autumn season 97%-99% of the annual loads of total nitrogen, total phosphorus, and suspended solids was acquired from the cultivated basin and 89%-91% from the forested basin. During the same seasons 99% and 87% of the total armual runoffs were recorded in the cultivated and forested basins. This means that in only 5 months of the year more than 95% of the nutrient and water rimoff occurred in the cultivated catchment, and about 90% in the forested catchment. Thus the values of nonpoint loads, normally presented as... 

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