Table 1. Sample Calculations of Direct Photolysis HalfLife of Carbaryl in Water Midday, Midsummer, Latitude 40°N Wavelength, nm
297.5 300 302.5 305 307.5 310 312.5 315 317.5 320 323 330
CA
1480 918 74 1 532 427 356 388 26 1 235 101 45 11
Z X x 10-14
0.00716 0.0240 0.0723 0.181 0.305 0.495 0.717 0.933 1.15 1.35 2.52 8.46
~ A Z Ax 10-15
1.06 2.20 5.36 9.63 13.0 17.6 27.8 24.3 27.0 13.6 11.3 9.31
FxZx = 162 2.303 x 1.62x 1017 = 6.20 x 10-4 s-1 6.02X lozo $313 = 0.0060 0.693 = 1.86 X lo5 s = 51.7h f112 = (0.0060X 6.20X k, =
(5) Lamola, A. A., Hammond, G. S., J . Chem. Phys., 41, 2129 (1965). (6) Zepp, R. G., Wolfe, N. L., Gordon, J . A,, Fincher, R. C., J . Agric. Food Chem., 26,727 (1976). (7) Riddick, J. A., Bunger, W. B., “Organic Solvents, Physical Properties and Methods of Purification”, Wiley-Interscience, New York, N.Y., 1970. (8) Strickler, S. J., Berg, R. A., J . Chem. Phys., 37,814 (1962). (9) Schwarz, R. P., Wasik, S. P., Anal. Chem., 48,524 (1976). (10) Jaff6, H. H., Orchin, M., “Theory and Applications of Ultraviolet Spectroscopy”, pp 186-8, Wiley, New York, N.Y., 1962. (11) Moses, F. G., Liu, R.S.H., Monroe, B. M., Mol. Photochem., 1, 245 (1969). (12) Zepp, R. G., Wolfe, N. L., Azarraga, L. V., Cox, R. H., Pape, C. W., Arch. Enuiron. Contam. Toxicol., 6,305 (1977). (13) Wolfe, N. L., Zepp, R. G., Baughman, G. L., Fincher, R. C., Gordon, J. A,, “Chemical and Photochemical Transformation of Selected Pesticides in Aauatic Svstems”, EPA Report No. EPA600/3-76-067, Sept. 1976: (14) Zepp, R. G., unpublished results. (15) Smith. J. H.. Mabev. W. R.. Bohonos. N.. Holt. B. R.. Lee. S. S.. Chou, T . - W . ,Bomberger, D. C., Mill, T., “Environmental Pathways of Selected Chemicals in Freshwater Systems”, EPA Report No. EPA-600/7-77-113, Oct. 1977. (16) Parker, C. A., in “Advances in Photochemistry”, W. A. Noyes, G. S.Hammond. and J. N. Pitts. Vol 11. DD 308-9. Interscience, London, England, 1964. (17) Ross, R., Crosby, D. G., Chemosphere, 4,277 (1975). 1.
Received for review May 6,1977. Accepted August 29, 1977.
Simple Apparatus for Monitoring Land Disposal Systems by Sampling Percolating Soil Waters Craig D. Stevenson Chemistry Division, DSIR, Private Bag, Petone, New Zealand
T h e treatment efficiency achieved by land disposal of effluents may conveniently be assessed by collection and analysis of waters percolating through unsaturated soil zones underlying treatment plots. A simple and inexpensive suction sampler constructed from “Swinnex”-type filter holders, membrane filters, and glass fiber filter discs is described. Air entry and vacuum loss occur only a t high soil-moisture tensions; therefore, the system will sample soil waters over relatively long periods aft.er a single evacuation. Loss of sample under high soil-moisture tensions is avoided by collecting the sample in a container separate from the sampling probe. The sampler materials cause minimal (if any) changes in sample composition. The efficiency of conventional treatment processes is usually assessed by comparing the composition of wastewater before and after passing through the treatment process. Use of this approach for irrigation or infiltration/percolation modes of land disposal treatment requires the collection of a “treated” sample that may be compared directly with the applied effluent. This can usually be achieved by collecting waters percolating through unsaturated soil zones underlying the treatment plot, where predominantly vertical water movement can be assumed. Collection of samples from the saturated zone (e.g., by wells) is often unsuitable for assessing treatment efficiencies, because this zone will usually contain water originating both 0013-936X/78/0912-0329$01 .OO/O
@ 1978 American Chemical Society
from the applied effluent and from rain or irrigation waters infiltrating in other areas and moving laterally under the treatment plot. The proportions of mixing of waters from the different possible sources are seldom known. Installation of drains for the purpose of monitoring a land disposal site is not recommended, because “short circuiting” of relatively poor quality effluent into the drains is likely ( I , 2). Porous ceramic cups have been used to sample waters percolating through unsaturated soil zones for a variety of investigations including assessment of the effectiveness of systems for land application of effluents ( 3 ) and movement of nitrates through soils ( 4 , 5 ) . In a typical application the sampler is inserted into a tightly fitting hole bored in the soil profile and then evacuated to a pressure that will withdraw water present in the soil profile under soil-moisture tensions in the range of interest. The water sample collects in the ceramic cup and is withdrawn after removing the bung sealing the tube to which the porous cup is connected. Porous ceramic cup samplers are often unsuitable for determination of phosphorus concentrations in soil solution because of adsorption/desorption reactions that can cause significant changes in concentrations of this element in sampled solutions. For example, one ceramic cup examined a t this laboratory released phosphorus to distilled water passed through it, giving concentrations of ca. 0.025 g/m3 P. A similar but smaller problem may exist for ammonium ion and other cations. A further disadvantage of some porous cup samplers is associated with the storage of sample in the porous cup, in that Volume 12,Number 3,March 1978
329
when soil-moisture tensions exceed the applied vacuuII1, the sample is withdrawn from the sampler into the soil. An all-plastic suction sampler not subject to significant phosphorus adsorption/desorption problems has been described (6).However, this sampler is subject to entry of air a t relatively low soil-moisture tensions and can therefore only be used under conditions of high soil moisture and with constant pumping. This communication describes a suction sampler constructed from readily available plastic and glass fiber components. Soil waters may be sampled over relatively long periods after a single evacuation of the sampler, and little sample water collected is lost a t high soil-moisture tension.
filter support / b a s e membrane filter sealing shoulder cut COP
Figure l a . Preparation of “Swinnex”4ype filter holder for suction
sampler
filter support / base membrane filter glass-fibre prefilter s-fibre ‘wick ’
Construction of Sampler The suction sampler is based on “Swinnex”-type (Millipore Corp.) filter holders, often used for filtration of fluids delivered by syringe. T o prepare the filter holder for use in the sampler, the cap is cut so as to remove much of the dome, but leave the sealing shoulder (or O-ring groove in some types) and threaded portion intact (Figure l a ) . A polycarbonate (0.40.8-1m pore size) or cellulose acetate membrane filter (0.45-5-~mpore size) and a glass fiber prefilter (e.g., Whatman GF/C or GF/F) are fitted to the holder, and a fine-bore sampling tube is attached to the filter support/base. Filter holders for 25-mm membranes are convenient, but other sizes (preferably larger) may be used. T o install the suction sampler, a hole (typically 10 cm diameter) is dug to an appropriate depth in the soil, using an auger or coring tube. Three or four sheets of 9- or ll-cm-diameter Whatman GF/C glass fiber filter are placed in the bottom of the hole, and two or three smaller glass fiber “wick” discs that fit within the hole in the filter holder cap are placed centrally on the sheets (Figure lb). The filter holder is placed so that the smaller glass fiber “wick” discs contact the prefilter; while the filter holder is held down against the collector sheets, some of the finer material removed from the hole is replaced and tamped lightly to retain the filter holder in place. During back-filling of the hole, it is desirable to include some layers of decreased permeability to encourage the movement of percolating waters out of the disturbed core above the suction sampler and into the surrounding undisturbed profile. Depending on circumstances, this may be achieved by tamping the backfill a t intervals or by inserting polythene discs a t various depths in the backfill. The sampling tube passes to the surface and is connected to a sample receiver as illustrated in Figure 2. A Buchner flask (250 or 500 mL) is a convenient sample receiver. A 2-L glass bottle covered with glass fiber-reinforced tape is used as a vacuum reservoir. The vacuum indicator consists of a small bore (3 mm) glass tube into which a segment of water colored with copper sulfate is introduced. The volume (length) of the air trapped by the water segment is inversely proportional to the pressure in the sampler, and the device provides adequate indication of the sampling vacuum at pressures down to approximately 0.5 bar. In operation the pressure in the sampling system is usually reduced to approximately 0.6 bar, corresponding to soilmoisture tensions above which many soils drain only slowly. This is achieved by evacuating the vacuum reservoir to approximately 0.5 bar in the laboratory, connecting it to the sample receiver in the field, and then releasing the vacuum reservoir clamp. In practice, collected waters will often stand in the sample receiver for several days, and sample preservation is necessary. Mercuric chloride solution is added to the receiver to give a concentration of 40 g/m3 HgClz when the receiver is full. A number of studies have demonstrated the effectiveness of 330
Environmental Science & Technology
glass-fibre’ collector”
Figure lb. Installation of suction sampler showing glass-fiber “wick”
and “collector” arrangement scale markings
undisturbed soil
collector filter sheet
Figure 2. Suction sampler installation, sample receiver, and vacuum
indicator mercuric chloride as a preservative for samples to be analyzed for nitrogen and phosphorus constituents (7-9). Discussion Once installed, the sampler withdraws water from the soil profile a t a slow rate (ca. 1-15 mL/h) when the soil moisture tension is below the applied vacuum in the sampler. The rate of sample collection appears to be controlled predominantly by the hydraulic conductivity of the soil a t the time of sampling. Relatively high sampling rates are obtained under moderately moist soil conditions, when relatively high permeability and low soil moisture tensions would be expected. Low sampling rates occur under drier conditions when permeabilities would decrease and soil moisture tensions increase. When the soil-moisture tension rises above the applied vacuum, only the small sample volume in the sampling tube and filter holder is withdrawn into the soil. So long as the membrane filter is moist, the small pore size and hydrophilic character of the membrane prevent passage of air through the membrane; therefore, no entry of air, and consequent vacuum loss, occurs. When the soil is wetted again, further water is collected. Under very dry soil conditions the membrane dries out, and rapid vacuum loss occurs. In practice, these suction samplers have been found to retain their vacua for a period of about two weeks after a single thorough soaking of a very freely drained
pumice soil, when fitted with 5-hm pore membranes. Membranes having smaller pores would presumably extend this period. In early tests the “Swinnex” holders, fitted with a membrane and prefilter, were installed without the glass fiber “wick” and “collector”. Under these conditions, rapid blockage of the membrane occurred, and the sampling rate became unacceptably low (ca. 0.2 mL/h). The “wick and collector” system gives capillary contact with a relatively large area of soil; even where the “collector” becomes blocked by fine soil particles, a satisfactory sampling rate is maintained. Obviously, the sampling area and rate can be increased by using a larger “collector” sheet. In general, it is preferable to use a membrane filter having a larger pore size than the effective pore size of the glass fiber “collector”, to restrict blockage to the large area of the “collector”. I t is possible that microbiological growths could be established on the membrane filters (particularly on cellulose acetate membranes) and limit the useful life of the samplers by filter blockage or breakdown, or sample contamination. Since the polycarbonate membranes are made of a relatively nonbiodegradable material, they are generally preferred. Wagemann and Graham ( I O ) have shown that cellulose acetate membranes and glass fiber filters cause little, if any, changes in concentration of dissolved constituents during filtration. Tests a t this laboratory have shown that polycarbonate membranes (Johns-Manville “Membra-fil”) did not add detectable amounts of ammonia, nitrate, organic nitrogen, or total phosphorus to 300 mL of distilled water that passed through the unwashed membranes. When two synthetic samples containing concentrations of NH3-N, NO:3-N, and organic-N each in the range 0.05-0.33 g/m3 and total phosphorus in the range 0.005-0.01 g/m3 were passed through the distilled water-washed membranes, the observed concentration changes did not exceed the experimental errors of the analyses (5% or f0.005 g/m3 N for ammonia, 1%or f0.001 g/m3 N for nitrate, 10% or f0.02 g/m3 for organic-N and f O . O O 1 g/m3 for total phosphorus). In the cellulose acetate membrane samplers used by the author, possible microbiological problems have been minimized by precipitating silver chloride in the prefilters. Two drops of silver nitrate solution ( 1 ~ 5 % w/v) were placed on each prefilter, followed by two drops of sodium chloride solution (1-596 w/v). After precipitation of the silver chloride, the filter was washed thoroughly with distilled water. The equilibrium solubility of silver chloride maintains silver concentrations of approximately 0.01-0.1 g/m3 in solutions, and these concentrations are bactericidal and fungicidal ( I 1 ). The silver chloride precipitate should be sufficient to be effective during the collection of 10-300 L of sample water, depending on the quantity of the precipitate and the chloride concentrations in the soil. Under conditions of low chloride concentrations in soil water, the use of silver bromide may be preferable. Samplers treated with silver chloride have operated successfully for a period of 11 months a t the time of writing. Laboratory checks have shown that the silver chloride precipitates removed approximately 25% of the ammonia and
75% of the organic nitrogen from a solution containing 0.24 g/m“ “3-N and 1.1g/m3 organic-N. However, concentrations of nitrate (0.43 g/m3 N03-N) and reactive phosphorus (0.012 g/m3) were unchanged by the precipitate. These results suggest that silver chloride is a suitable membrane preservative in nutrient removal studies where nitrate is the predominant nitrogen constituent in soil solution, as often occurs in aerobic soils. Silver halide precipitates may not be suitable for use under anaerobic soil conditions. A deoxygenated solution containing 50 g/m3 ferrous ion and 20 g/m3 manganous ion was passed through a silver chloride-treated filter in the dark, a t a rate of 2 mL/h. The chloride concentration in solution increased from 18.5to 23.5 g/m3, and the ferrous ion concentration decreased by approximately 10 g/m3 relative to the same solution that had passed through an identical filtration system in the absence of silver chloride. This suggests that the reaction
Fef2
+ AgCl
-
Fe+3
+ Ag + C1-
may have occurred. Ferric oxide was observed on the silver chloride precipitate after the experiment, and this would adsorb phosphate and possibly a variety of cations from solution. The sampling system described has been used successfully a t depths of up to 1 m. In principle, it should be suitable for sampling a t depths up to about 4 m, but a t greater depths a modified sample receiver would need to be buried to overcome the head resulting from drawing up a water column in the sampling tube. A valve system, as described by Wood (12), would be required to withdraw sample from the receiver. For most land disposal systems, monitoring at depths greater than 1 m offers little advantage, since the predominant effluent renovation effect is usually obtained in the upper 0.3 m or so of soil.
Literature Cited (1) Searle, S . S., Kirby, C. F., Water Spectrum, 4 ( 3 ) , 15-21
(1972). (2) Karlen, D. L., Vitosh, M. L., Kunze, R. J., J . Enuiron. Qual., 5 (3), 269-73 (1976). ( 3 ) Dugan, G. L., Young, R.H.F., Lau, L. S.,Ekern, P. C., Loh, P.C.S., J . Water Pollut. Control Fed., 47 (8),2067-87 (1975). (4) Wagner, G. H., Soil Sci., 94 (61,379-86 (1962). (5) Wagner, G. H., ibid., 100 (61,379-402 (1965). (6) Quin, B. F., Forsythe, L. J., N . 2. J . Sci., 19 (2), 145-8 (1976). ( 7 ) Henriksen, A., Vattenhygien, 25,247-54 (1969). (8) Hellwig, D.H.R., Water Res., 1 ( l ) ,79-91 (1967). (9) Bowitch, D. C., Edmond, C. R., Dunstan, P. J., McGlynn, J. A,, “Suitability of Containers for Storage of Water Samples”, Australian Water Resources Council, Technical Paper No. 16, Canberra, Australia, 1976. (10) Wagemann, R., Graham, B., Water Res., 8 (71,407-12 (1974). (11) McKee, J. E., Wolf, H. W., “Water Quality Criteria”, California State Water Resources Control Board Publication 3-A: 256-7, 1971. (12) Wood, W. W., Water Resour. Res., 9 (2), 486-8 (1973).
Received for review April 25, 1977. Accepted September 12,1977.
Volume 12, Number 3, March 1978 331
