Original Research Paper n

Chemometrics and Intelligent Laboratory Systems, 3 (1988) 73-78 Elsevier Science Publishers B.V.. Amsterdam - Printed in The Netherlands

Spatial Resolution of Groundwater Contamination by Soil-Gas Measurement

H.B. KERFOOT and M.J. MIAH *

Lockheed Engineering and Management Services Company, Inc., Las Vegas, NV 891 I9 (U.S.A.)

ABSTRACT

Kerfoot, H.B. and Miah, M.J., 1988. Spatial resolution of groundwater contamination by soil-gas measurement. Chemometrics and Intelligent Laboratory Systems, 3: 73-78.

An empirical determination of the spatial resolution of soil-gas measurement in detection of subsurface contamina-

tion by volatile organic compounds (VOCs) was performed. Soil-gas measurements were made above chloroform-

contaminated groundwater. The precision of the technique was assessed by evaluation of the variance of results from

closely spaced samples and was found to vary with the soil-gas VOC concentration. The spatial resolution of the

technique was determined on the basis of the precision of results and also varied with soil-gas chloroform concentra-

tion. The spatial resolution ranged from 13 to 86 feet above chloroform groundwater concentrations of 28 to 555 pg/l,

respectively. The observed spatial resolution depended on both the soil-gas concentration and the concentration

gradient. The best spatial resolution obtained was out at a low concentration and high concentration gradient and was approximately the depth to the groundwater source.

INTRODUCTION

Control of contamination of groundwater by organic compounds due to improper waste dis- posal and from leaking underground storage tanks is a national priority. Federal and private re- sponses to the delineation of such contamination typically involve drilling exploratory boreholes, obtaining groundwater samples, and sending the samples to the laboratory for analysis. Choices of the locations of these boreholes are often made based on limited data, due to a dearth of informa- tion about the migration and extent of the con- tamination. Because of this, efforts have been made to develop surface reconnaissance tech- niques for detection and measurement of sub- surface contamination [l]. The results of such

surveys can be used to plan more cost-effective strategies for site-characterization efforts.

Soil-gas measurement is an emerging technol- ogy for indirect detection and delineation of sub- surface contamination by volatile organic com- pounds (VOCs) [2]. In this technique, soil gas is sampled from the vadose (unsaturated) zone overlying groundwater and analyzed for VOCs.

As with any measurement technique, workers must know the performance characteristics of soil- gas surveying to make efficient use of the technol- ogy. To design a cost-effective sampling network, sampling locations should be far enough apart to provide information that is not redundant, or is significantly different in a statistical sense. In this paper, we use the functional relationship between the overall system precision of a soil-gas surveying

0169-7439/88/$03.50 0 1988 Elsevier Science Publishers B.V. 73

l Chemometrics and Intelligent Laboratory Systems

technique and the measured soil-gas concentration at sampling locations, along with linearly inter- polated soil-gas concentrations between sampling locations to estimate the minimum distance from each sampling location where a statistically sig- nificantly different concentration would be mea- sured. We call this distance the spatial resolution of the technique.

The concept of spatial resolution is a familiar one in remote sensing technology. Optical spatial resolution has been defined as “the minimum distance between two objects that a sensor can record distinctly” [3]. The particular sensor system used determines what particular mathematical for- malism is appropriate to calculate the spatial reso- lution; for optical sensors, the Rayleigh criterion is used [4,5]. This criterion considers the processes which affect propagation of electromagnetic radia- tion to be the ones that determine spatial resolu- tion.

In a first approximation, VOC vapors emanat- ing from a particular subsurface point diffuse isotropically. Based on this simplistic model of isotropic VOC diffusion, the minimum horizontal separation between surface soil-gas sampling loca- tions where significantly different concentrations could be measured would depend on many fac- tors. Subsurface heterogeneity would make model- ing the situation even more difficult. For that reason we have performed an empirical calcula- tion of the spatial resolution of a soil-gas survey technique.

EXPERIMENTAL

The soil-gas measurement technique used in this study is based on a charcoal sorbent sampler, buried at a l-foot depth, which is solvent-de- sorbed and analyzed by gas chromatography [2]. Field blank results were subtracted from all soil- gas measurements. Groundwater samples were analyzed by a U.S. EPA gas chromatography-mass spectrometry method [6], and grab samples of soil gas were analyzed in a study described elsewhere

111.

SITE DESCRIPTION

The survey was carried out at the Pittman Lateral in Henderson, Nevada (Fig. 1). The Pitt- man Lateral is a right of way with groundwater monitoring wells at 200-foot intervals along a line perpendicular to the northward flow of ground- water. The location has been used for earlier soil- gas studies [l].

The site is located in one of the lowest areas of the Las Vegas Valley. Site elevation ranges from 1600 to 1700 feet above mean sea level. The materials of the vadose zone and of the aquifer are poorly sorted to moderately stratified sediments with a low porosity and moderate permeability. Soils developed on these materials can have well- developed calcic horizons as much as 20 inches (50 cm) thick. Locally, soils do not have well-devel- oped horizons and have negligible organic con- tent. A clay layer up to 15 feet (4.5 meters) in thickness underlies the aquifer and retards down- ward groundwater movement. A buried paleo- channel trends north-south through this clay layer located between wells 630 and 635 (Fig. 2) and extends to a 60-foot (1%meter) depth below the land surface. Groundwater flow is to the north towards the Las Vegas Wash and nearby Lake Mead.

Chloroform is present at concentrations up to 1000 pg/l in groundwater samples on the eastern side of the Pittman Lateral from wells 621 to 629 (Fig. 2). On the western side of the Lateral, a groundwater plume of benzene, chlorobenzene, and non-volatile organic compounds is present. Measurements in this study were made above the chloroform plume. Table 1 lists the chloroform concentrations measured in groundwater samples from the site.

SAMPLING LOCATIONS

Soil-gas samplers were placed at a depth of 1 foot, 20 feet upgradient (south) of six wells, as shown in Fig. 3. The sampling locations included groups of four samplers placed in a 3-foot-square pattern, with the northeast comer of each square 20 feet to the south of the well at five of the wells

74

Original Research Paper n

LAS VEGAS WASH

- PITTMAN LATERAL TRANSECT

NEVADA

Fig. 1. Location of the study site.

(621 through 629). These locations were chosen to nique [l] and to provide an estimate of the overall duplicate sampling locations from an earlier soil- precision of the technique. Table 1 lists each sam- gas survey at the site using a grab-sample tech- pling location in this study and in the earlier

TABLE 1

Analytical results for chloroform

Well No. Groundwater Soil gas

TMrcrcfzraon Passive sampling Grab sampling

(ng/l) * @g/p 1) (ppbv) *

629 11 11.00 31

9.31

10.79

8.30

627 175 28.13

26.42

19.96

23.81

625 866 68.20

43.96

52.31

55.48

623 555 12.51

8.46

7.28

10.44

621 28 0.112

0.153

0.146

0.143

46

511

27

10

Field blank

* Source: ref. 1.

0.18

CHLOROFORM 3 =

1660- i=

2 -505

500 fi SURFACE

, _/---

g&&i ;

Fig. 2. Subsurface geology at the Pittman Lateral.

75

n Chemometrics and Intelligent Laboratory Systems

8 8 8 8 8 8 . . . . . . 631* 629 627 625 623 621

9 WELLS

l SAMPLING LOCATIONS

: : T u .9m : : 1 GROUNDWATER FLOW NORTH

* 2 LOCATIONS NOT SAMPLED AT 631

SCALE

Fig. 3. Sampling locations at the Pittman Lateral.

soil-gas study at the site, along with the respective soil-gas measurement results at each. In addition, the locations of groundwater monitoring wells and the measured chloroform concentrations at each are listed.

RESULTS

The groundwater chloroform concentrations, the soil-gas chloroform concentration at a 4-foot depth, and the amount of chloroform collected by passive sampling correlate (Table 1) with each other. Fig. 4 shows all three data sets. The correla- tion coefficient between the mean passive-sam- pling soil-gas analysis results at each well and the groundwater measurements is r = 0.78 (n = 5) and the correlation coefficient between the mean of the other soil-gas results and the groundwater measurements is r = 0.81 (n = 5). The two soil-gas data sets correlate, with r = 0.94 (n = 5).

For statistical analysis, results from Table 1 were used. From the standard deviations of groups of four samplers in a 3-foot-square pattern, it was observed that the precision of the technique depended on the concentration of chloroform in soil gas at a given sampling location. The standard deviation of results is described by the following equation (with r = 0.989, n = 5):

Si = -0.14 + 0.18X, (I)

where Xi = mean soil-gas concentration at loca- tion i.

Based on the observed relationship between the measurement precision and linearly interpolated values of adjacent soil-gas concentrations, the nearest soil-gas concentration, Xj, which differs significantly from Xi can be estimated:

2(X. - X,)/( S,’ + Sz)1/2 > 2.132 (2)

Fig. 4. Groundwater and soil-gas chloroform concentrations measured at the Pittman Lateral. (0) Groundwater chloro-

form concentration (gg/l); (a) soil-gas passive-sampling re-

sults (ng/pl X 10); (0) soil-gas grab-sampling results (ppbv).

76

Original Research Paper n

where Si and Sj are the predicted standard devia- tions of the observed (Xi) and predicted (Xi) mean values. The value of 2.132 is the value of t at the 95 percent level of significance with 4 degrees of freedom. Eq. 2 can be used to predict the value of Xj which will be significantly differ- ent from Xi. In the case where eq. 2 is an equality, or at the minimum distance to a significantly, different concentration from Xj, Si can be de- termined using eqs. 1 and 2. This minimum dis- tance is the spatial resolution at location i. Table 2 shows the results of such calculations from these data.

The model used, as described by eqs. 1 and 2, should be considered an empirical one. Eq. 2 may not be appropriate for the distribution of S$; in that case 2( Xi + Xj)/(S,” + ,S”)1/2 is not an exact t.

From the values in Table 2 it can be seen that the observed spatial resolution of groundwater contamination detection by soil-gas measurement is related to concentration. This observation agrees with the concept of horizontal diffusion limiting spatial resolution, since horizontal diffusion is di- rectly proportional to the horizontal concentration gradient. At higher concentrations at this site, larger concentration gradients exist and greater horizontal diffusion is expected. It may also be noted that the best spatial resolution obtained, 13

TABLE 2

Estimated and observed soil-gas concentration, standard devia- tion, and spatial resolution values

Soil-gas Standard

concen- deviation

tration (r)

Location (ft. from well 623)

Spatial resolution

(ft.)

kit/l) 9.85* 1.63 0.00 39.10

12.73 2.16

18.83 3.25

24.58* 4.30

32.58 5.67

41.89 7.40

54.89* 9.80

41.87 7.43

12.49 2.10

9.67* 1.60

39.10 _

121.00 79.00

200.00 52.00

252.00 314.00 86.00

400.00 57.00 457.00 _

587.00 13.00 600.00 _

l Observed soil-gas concentration (pg/l); other data inter-

polated.

feet, is approximately the depth to groundwater as would be expected for isotropic diffusion from a point source at a depth of 13 feet.

DISCUSSION

Soil-gas sampling and analysis results showed good correlation with groundwater contamination and can be a valuable site-reconnaissance tool. Based on results from this site, soil-gas measure- ment showed a spatial resolution which depends upon both the soil-gas VOC concentration and concentration gradient. However, the form of this relationship has not been described at this time and may well be influenced by several parameters not measured in this study, such as vadose-zone porosity or depth to groundwater. Both of these parameters will be site-specific, depending on the anisotropy of VOC diffusion in the soil [7]. In addition, the spatial resolution obtained for surface sensing of groundwater contamination by VOCs will be strongly influenced by the depth to the groundwater.

Further work to devise an analytical expression for the spatial resolution of soil-gas sensing is being performed. Further studies under varying geohydrologic and geochemical circumstances are needed to better define this concept.

ACKNOWLEDGEMENTS

The authors would like to acknowledge the assistance of C.L. Mayer of Lockheed-EMSCO in data collection and discussions with L.J. Blume of the U.S. EPA in Las Vegas about soils.

Although this work was supported in part by the U.S. EPA, it has not undergone review by that Agency and does not reflect Agency policy.

REFERENCES

H.B. Kerfoot, Shallow-probe soil-gas sampling for indication of groundwater contamination by chloroform, Internafianal Journal of Environmental Analytical Chemistry, 30 (1987) 167-181.

77

n Chemometrics and Intelligent Laboratory Systems

H.B. Kerfoot and C.L. Mayer, The use of commercial in-

dustrial-hygiene samplers for soil-gas surveying, Ground

Water Monitoring Review, Vl(4) (1986) 74-78.

D.S. Simonett, The development and principles of remote

sensing, in D.S. Simonett and F.T. VIaby (Editors), Manual of Remote Sensing, Vol. I, American Society of Photogram-

metry, Falls Church, VA, 1983, p. 20.

F.H. Perrin, The structure of the development image, in T.H.

James (Editor), The Theory of the Photographic Process, MacMillan, New York, 1966, pp. 499-551.

P.N. Slater, A re-examination of the Landsat remote sensing,

in R.A. Reeves (Editor), Manual of Remote Sensing, Ameri-

can Society of Photogrammetry, FaIIs Church, VA, 1979, pp. 235-251.

6 U.S. EPA, Methoak for Organic Chemical Analysis of Muni- cipal and Industrial Wastewater, Method 624, EPA-6001

4-82-057, U.S. EPA, Cincinnati, July 1982.

7 E.P. Weeks, D.E. Earp and G.M. Thompson, Use of atmo-

spheric fluorocarbons F-11 and F-12 to determine the diffu-

‘

sion parameters of the unsaturated zone in the southern

High Plains of Texas, Water Resources Reseurch, 18 (1982)

1365-1378.

78