field determination of chloride in salt impacted soils – just add ... - IPEC

FIELD DETERMINATION OF CHLORIDE IN
SALT IMPACTED SOILS – JUST ADD WATER!
David G. Boyer, P.G., Safety and Environmental Solutions, Inc., Hobbs, New Mexico
ABSTRACT
The standard method for determination of chloride in soils has been modified for
quick determination of chloride in the field using commercially available chloride test
strips instead of laboratory chemicals. The method utilizes a portable scale to obtain a
weighted sample of dry material that is placed in a 250 mL plastic bottle. Distilled water
is added to the bottle and it is shaken vigorously for several minutes. The mixture is
allowed to settle for a few minutes to reduce turbidity and decanted into a funnel lined
with a piece of filter paper. The filtered liquid is collected in a small jar or 40 mL vial
that holds the chloride test strip. The strip requires only a few drops of the filtrate, which
is absorbed by capillary action. A quick calculation converts the concentration shown on
the strip to mg/Kg (ppm). The entire test takes less than ten minutes and results are
comparable with laboratory results. Field and laboratory results for a number of sites in
the vicinity of Lovington, New Mexico, are presented.
1

INTRODUCTION
Water produced concurrent with the extraction of crude oil or natural gas
commonly contains chemical constituents with concentrations many times those found in
fresh water or even sea water. When introduced to the surface environment through
planned or unplanned releases produced water will adversely affect or destroy surface
vegetation and can potentially contaminate fresh groundwater supplies.
In the past, common disposal of produced water was by placement in unlined
surface impoundments or pits. For example, in rural Lea County New Mexico south of
Hobbs, approximately 29 million barrels of brine were produced in 1955 with most
disposal to unlined pits (1). Beginning in the 1950’s, the state regulatory agency, the Oil
Conservation Division, instituted restrictions on disposal to unlined pits in southeastern
New Mexico and today, with some exceptions, such disposal is prohibited statewide.
Although surface disposal is generally prohibited, produced water still can
impact the environment through accidental releases from leaks or breaks in flowlines,
tank failure, or from transportation accidents.
Whether the concern is environmental effects from old disposal pits or from
accidental releases, it is important to delineate the area affected to assess impacts to
surface resources and the potential for groundwater contamination. Following such
evaluation, the information collected is used as a basis for decision making for physical
clean up or to allow risk-based closure of the pit or spill location.
Aside from obvious visible surface impacts such as dead vegetation or salt
deposits, delineation of chloride impacts from produced water can be difficult without
laboratory analytical testing. Some test kits are available that use reagent chemicals such
as silver nitrate and require mixing and titration to determine chloride concentration in
the solution. Such kits are difficult to use in the field for chloride determination in water,
let alone soil.
Recently, the Hach® Company has produced two chloride test strips (Quantab®
High and Low Range) that allow testing of the chloride concentration in water by merely
dipping a strip in water and waiting a few minutes for capillary action to saturate it.
Following saturation, the strip is read and the corresponding chloride concentration is
found using a chart printed on the bottle. The method has been adapted to enable
environmental personnel to quickly and inexpensively determine chloride concentration
in soil in the field using readily available pieces of equipment.
2

PROCEDURE DESCRIPTION
A generalized description of the procedure is provided in this section. A detailed
list of instructions, suitable for providing to field technicians, is provided in the Appendix
at the end of this paper.
One method by which soluble salts in soils such as chloride can be determined or
estimated is from measurements made on aqueous extracts of soil samples (2). Other
methods may be more useful for determination of soil salinity and its relation to field soil
water content necessary for crop growth, but are not relevant in this instance. Aqueous
extracts on the order of 1:4 or 1:5 weight to volume commonly are used by analytical
laboratories for chloride determination in soil. The procedure described in this paper uses
a 1:4 extract.
Necessary Equipment
A minimum of special equipment is needed to perform the field determination of
chloride. A stainless steel spoon and mixing bowl are commonly used in collecting soil
samples. Additional items are a portable battery operated scale (0 to 200 gram range),
disposable weighing dishes, a 250 mL wide mouth plastic bottle, a small graduated
cylinder, a plastic funnel, filter paper, a small jar or 40 mL vials, distilled water and the
Hach Quantabs®. The Quantabs come in high and low range strips with chloride
concentration ranging from 300 to 6000 ppm and 30 to 600 ppm, respectively. All
reusable equipment should be clean before use; the funnel and small jar or 40 mL vials
must be completely dry. The equipment is carried in a foam-lined hard plastic case (e.g. a
Pelican Case®) for protection against vibration and moisture.
Sample Preparation
Samples are commonly collected from the soil surface or from boreholes drilled
with a hollow stem auger equipped with a split-spoon or core barrel sampler. The sample
is placed in a mixing bowl and organic matter such as roots discarded. Rock and gravel
larger than small pebbles are commonly removed. Homogenize the sample by thoroughly
mixing it prior to weighing or placement in a sample jar (if laboratory analysis is to be
performed). To the extent possible the material placed in a sample jar for laboratory
analysis should be as much like the sample selected for field determination. In addition
coordinate with the analytical laboratory to ascertain if they have a standard protocol for
selection of small volume samples (e.g. a maximum size of pebbles in the sample).
A tared weighing dish is placed on the scale and 25 grams of the sample is
weighed and then placed in the plastic bottle. 100 mL of water is added and the mixture
shaken for one minute or longer. Allow the sample mixture to settle for a short time
period so that heavy particles drop out. Fold a piece of filter paper into quarters and place
in the funnel. Place the funnel in the small jar or 40-mL vial together with a Quantab®
strip. Decant the sample into the folded filter paper and allow approximately ½ inch of
clear filtrate to accumulate in the vial before removing the funnel. Turbid solutions in the
vial will clog the capillary pores and cause very slow or incomplete reactions.
3

Chloride Determination
When a Quantab® strip is placed in an aqueous solution, fluid rises up the strip
by capillary action until the strip is completely saturated. The strip contains silver ions,
which combine with chloride in the sample to form a white column of chloride. A
moisture sensitive yellow string across the top of the tab turns blue-black when saturated
and indicates completion of capillary action and the reaction. The length of the white
silver chloride column on the strip is proportional to the chloride concentration.
Following saturation, the value on the tab is read and the chart on the back of the
appropriate bottle used to get the chloride value for the strip. Each Quantab® lot is
calibrated independently; the chloride concentration chart on the bottle from which the
strip was removed is used as values may differ from those of a previous bottle. Multiply
the chart value by four (4) to get the chloride concentration in soil in ppm (mg/Kg).
Depending on the time taken to shake the sample, decant and filter the liquid, and the
turbidity of the filtrate, the time required to conduct a single test is usually from five to
ten minutes.
COMPARISON WITH LABORATORY RESULTS
The field results are compared with laboratory results for four sites located in the
vicinity of Lovington, New Mexico (Tables 1 through 4). The geologic setting is the
Tertiary Ogallala formation which is chiefly a calcareous, unconsolidated sand but with
some zones of sandstone of varying hardness (1). In the upper ten to twenty feet in the
study areas the formation is capped by a layer of caliche of varying thickness and density.
The borehole splitspoon/core barrel samples taken near the surface usually are
quite rocky and prior to testing the samples must be prepared as described above to
remove pieces of caliche and sandstone rock as big a 3-in. in diameter. Notwithstanding
sample preparation problems, the laboratory results show good agreement with the field
tests.
Table 5 presents the results of the comparison for the 62 samples tested.
Twenty-three samples (37 percent) of the field results were within 0 to 10 percent of the
laboratory results and another 14 samples (23 percent) were between 11 and 20 percent.
A total of 37 field samples (60 percent) were within 20 percent of the laboratory value.
Only 4 field samples (6 percent) exhibited greater than 50 percent difference from the
laboratory result. The highest percent difference between field and laboratory test results
was 63 percent.
Unlike water samples, soil samples are rarely homogenous with varying particle
size and with soluble constituent concentrations dependent on the initial soil water
concentration, the pathway followed in the porous medium, and the amount of sorption
on the soil particles. Given this innate variability, which exists even within a single soil
sample, agreement between field and laboratory results is consistent and certainly
acceptable for purposes of field delineation of chloride impacts from the types of releases
described in this paper.
4

COST CONSIDERATIONS
The cost per chloride test is minimal. A bottle of 40 Quantabs® available from
the Hach Company is currently priced at $32; therefore, each strip costs $0.80. The scale
used to weigh the soil samples is an Ohaus model CS200 available from
www.scalesonline.com at a cost of $55. The other test equipment is available from
laboratory supply companies at minimal cost.
SUMMARY
A quick and inexpensive test for field determination of chloride in salt impacted
soils has been developed and has proven to be very useful in the delineation of produced
water releases from oil and gas operations. The test requires a minimum of equipment to
perform and does not require the use of chemicals in the field, only distilled water. The
test can be completed within five to ten minutes depending on the turbidity of the
aqueous extract and the time required for settling and filtering the sample. Results from
the test are quite comparable with results from laboratory tests performed on the same
sample. Field results for sixty percent of the samples examined were within twenty
percent of the laboratory results. Although the cost per test is very inexpensive, the real
value of the test is the time saved on site by being able to directly delineate horizontal
and vertical extent of a release without the added expense of having outside contractors
return to a site for additional investigation.
REFERENCES CITED
1. Nicholson, A., Jr. and Clebsch, A. Jr., Geology and Ground-Water Conditions in
Southern Lea County, New Mexico, Ground-Water Report 6, State Bureau of
Mines and Mineral Resources, New Mexico Institute of Mining and Technology,
Socorro, New Mexico (1961).
2. Rhoades, J.D., “Soluble Salts,” in Methods of Soil Analysis, Part 2. Chemical
and Microbiological Properties, Chapter 10, American Society of Agronomy −
Soil Science Society of America Agronomy Monograph no. 9, Madison,
Wisconsin (2 nd Edition, 1982).
3. Franson, M. H. (editor), Standard Methods for the Examination of Water and
Wastewater, Seventeenth Edition, American Public Health Association,
Washington, DC (1989).
5

Table 1. Site 1, salt water line leak, northeast of Lovington, NM, north of US Hwy 82
Sample Location
Depth
(ft.)
Strip Range
(high or
low)
Field
Test
(ppm)
Laboratory
Analysis
(mg/Kg)
Percent
Difference from
Lab
BH-1 5 H 5,144 8,397 -38.7
BH-1 10 H 6,156 5,918 4.0
BH-1 15 H 3,856 3,679 4.8
BH-1R 18 H 5,632 6,958 -19.1
BH-1R (duplicate) 18 H 6,156 6,958 -11.5
BH-1R 22-24 L 372 496 -25.0
BH-2 5 H 2,800 3,759 -25.5
BH-2 10 L 1,816 2,719 -33.2
BH-2 15 L

Table 3. Site 3, leaky concrete-lined holding pond at salt water disposal facility,
northeast of Lovington, NM, south of US Hwy 82.
Sample
Location
Depth (ft.)
Strip Range
(high or low)
Field Test
(ppm)
Laboratory
Analysis
(mg/Kg)
Percent
Difference
from Lab
BH-1 5 L 680 976 -30.3
BH-1 10 L 1,384 1,456 -4.9
BH-1 15 L 636 736 -13.6
BH-1 20 L 212 528 -59.8
BH-2 10 L 280 576 -51.4
BH-2 15 L 436 720 -39.4
BH-2 20 L 960 1,136 -15.5
BH-2 25 L 1,200 2,143 -44.0
BH-2 30 L 1,956 2,399 -18.5
BH-2 35 H 3,292 3,538 -7.0
BH-2 40 H 2,326 2,799 -16.9
BH-2 45 H 2,108 2,623 -19.6
BH-3 15 L 1,592 976 63.1
BH-3 20 L 1,036 1,120 -7.5
BH-3 25 L 528 576 -8.3
BH-3 30 L 280 400 -30.0
BH-3 35 L 692 544 27.2
BH-3 40 L 392 368 6.5
BH-3 45 L 280 240 16.7
Table note: Chloride analysis using Standard Methods 4500-Cl - B.
7

Table 4. Site 4, tank overflows at salt water disposal facility, southeast of Lovington,
NM, west of NM Hwy 18.
Sample
Location
Depth (ft.)
Strip Range
(high or low)
Field Test
(ppm)
Laboratory
Analysis
(mg/Kg)
Percent
Difference
from Lab
BH-1 17-18 H 2,208 1,504 46.8
BH-1 24-25 H 1,944 1,344 44.6
BH-1 30 L 492 592 -16.9
BH-1 35 L 204 272 -25.0
BH-2 15 H 1,052 912 15.4
BH-2 19-20 L 824 928 -11.2
BH-2 24-25 L 368 464 -20.7
BH-2 30 L 148 208 -28.8
BH-3 10 L 492 496 -0.8
BH-3 15 L 700 720 -2.8
BH-3 20 L 540 464 16.4
BH-3 25 L 492 544 -9.6
BH-3 30 L 328 496 -33.9
BH-3 35 L 588 640 -8.1
BH-3 39-40 L 1,620 1,664 -2.6
BH-3 45 L 1,620 1,615 0.3
BH-3 55 L 1,620 1,711 -5.3
BH-3 60 L 2,020 1,951 3.5
Table note: Chloride analysis using Standard Methods 4500-Cl - B.
Table 5. Summary table showing relationship of field sample results to laboratory
analyses.
Percent Difference from Laboratory Result (±), n=62
0-10% 11-20% 21-30% 31-40% 41-50% >50%
23 14 11 5 5 4
37% 23% 18% 8% 8% 6%
8

Figure 1. Equipment setup for field determination of chlorides in soils.
Figure 2. Soil preparation and weighing of soil sample for chloride determination.
9

Figure 3. Hach® Quantab® test strips used for field determination of chloride in soils.
Figure 4. Hach® Quantab® chloride test strip at completion of chloride field test.
10

APPENDIX
Procedure for Field Determination of Chloride in Soil
Equipment:
4-oz. sampling jars, spoon, stainless steel mixing bowl, 250 mL plastic wide
mouth plastic bottle, 25 mL graduated cylinder, scale, disposable weighing
dishes, funnel, filter paper (Whatman® #4 Qualitative 125 mm diameter circles
or equivalent), 40 mL vials, Hach Quantab® strips (low and high range), distilled
water. Prior to use, all equipment should be clean and dry.
Sampling and preparation procedure:
Collect a soil sample from the surface or from a splitspoon/core barrel and place
in a mixing bowl and homogenize. Remove rocks and gravel larger than small
pebbles. Weigh 25 grams of soil and place in the wide-mouth bottle. For a 1:4
weight to volume dilution, add 100 mL of distilled water to the bottle. Shake
vigorously for several minutes (no longer than 5 minutes). Let the sample settle
for a minute or two to allow heavy particles to drop out. Fold and place filter in
the funnel and decant sample into the filter. Collect the filtrate in a clean 40 mL
vial. Only about ½ inch of clear filtrate in the vial is needed for the procedure.
Chloride Determination:
Place a high or low range Quantab® strip in the vial (replace cap on Quantabs
bottle to prevent moisture from degrading the tabs). Wait until the yellow strip at
the top of the tab turns blue-black. Read the value on the tabs and use the chart
on the back of the appropriate bottle to get the chloride value for each strip. Each
Quantab® lot is calibrated independently, so use the chart on the bottle where
you removed the strip; the chloride concentration chart may differ from a
previous bottle. Multiply the chart value by four (4) to get the concentration of
chloride in soil in ppm (mg/Kg). Record the value in field book.
For laboratory analysis, place sample from mixing bowl in a 4-oz jar and
transport to the laboratory with a chain-of-custody form.
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