Precipitation isotope collector designs
U.S. Geological Survey, Water Resources Division, 431 National Center, Reston, VA 20192, USA (703) 648-5890, mascholl @usgs.gov.
Various designs for isotope precipitation collectors are described below.
Precipitation collectors used in Hawaii studies (Scholl et al., 1996, Scholl et al., 2002) were designed for obtaining 6-month cumulative stable isotope samples. The primary goal was to avoid evaporation from the collector over this relatively long collection period. The collectors were placed in environments ranging from desert to rainforest. Three types of collectors were used in the original Hawaii Island study; on Maui, only the oil-type collectors were used. We currently use similar oil-type precipitation collectors in Puerto Rico, these are slightly different in design, and are described below.
1. Oil-type collector: evaporation is inhibited by a layer of oil in the collector, incoming water stays under the oil layer. At least a 1-cm layer of oil is required. Silicone oil and light mineral oil seem to work equally well; mineral oil is less expensive. The funnel contains a tuft of polyester fiber to filter debris. If a small-diameter funnel is used, the collection container requires an air outlet hole, or back pressure will not allow it to fill properly. Adapted from design by Friedman (1992).
Advantages: very easy to construct, high degree of success in obtaining unevaporated sample.
Disadvantages: Oil must be separated from samples and disposed of (see below). Animal damage, vandalism, or funnel getting knocked off may occur for all types of collectors.
2. Ball-in-funnel type collector: evaporation is inhibited by a hollow plastic ball placed in the funnel which floats when the funnel has water in it and covers the opening at other times. Funnel has a covering of wide mesh netting to keep out large leaves and other windblown debris. The container has no air outlet hole. This collector cannot be used in forested areas, debris falls into the funnel and interferes with function of the ball. Adapted from design by M.L. Davisson, oral comm., 1992. A variation on this design is one used by Kazahaya and Yasuhara (1994), where a ping-pong ball is situated in a chamber below the rain-collection funnel and above the collection container. This would presumably keep the ball from being taken out of the funnel, and debris might be less of a problem.
Advantages: clean samples, easy to construct.
Disadvantages: debris or insects get caught under the ball and leave an opening for water vapor to escape container. This happens frequently enough that replicate collectors should be placed to increase chances of an unevaporated sample. In places, mongoose or birds thought ball was an egg (?) and removed it, this may be a problem with other animals in other areas.
3. Bag and tubing collector: evaporation is minimized by 1) tubing extending from funnel stem to bottom of bucket - when bucket fills, only water in the tubing can evaporate from collector; and 2) a polypropylene bag inside the container encloses the tubing and sample runs into the bag - bag starts out flat and fills slowly leaving little headspace for water vapor to form inside container. Polypropylene bag shape was modified with a heat sealer so there was only a small opening for the tubing to go through. Bag and collector need tiny air outlet holes to fill properly.
Advantages: clean samples, can be used in forested areas.
Disadvantages: More work to construct than other types of collector. Over a long sample period, algae, etc. tends to grow in bag and tubing in forested areas where lots of debris falls into funnel, sometimes to the point of clogging tubing.
Construction and placement: Collectors were designed to collect as large a sample as feasible without overflow, and various size funnels were used for different rainfall areas. For Hawaii (up to 100 inches (2540 mm) rainfall in 6-month period) we used 3.5 or 5 gallon (13 or 19 liter) buckets, of the type used in the food service industry, with o-ring sealed lids. We used polypropylene or HDPE funnels set in the bucket lid and glued in place. A short section of PVC pipe was glued on the 2 inch (5 cm) diameter funnels (to make them like Buchner funnels). To glue plastic to plastic, use a thick glue with toluene, TCE or other solvent base. PVC cement and silicone-based sealants do not work well in outdoor conditions. Something needs to be placed in or over the funnels to filter debris if collector is in an area near trees (screen, fiber, mesh). We used plastic orchard netting with 1/2 inch (1.3 cm) mesh over the top of funnel for all designs, and additionally, a tuft of polyester fiber inside the funnel for the oil and bag type designs. The buckets were placed on the ground in wooden stands. Smaller-volume collectors can be mounted on posts to keep them off the ground. In this case, something needs to be done to keep birds from perching on the funnel. Some research groups bury the collection containers, with the funnel placed up on a post and connected with tubing to the container, in order to keep them protected from disturbance and high temperatures. Putting out several collectors in an area is advisable to assure that at least one sample is collected.
Animals and vandalism can be a problem; our best luck was with collectors that were well-hidden or on private property, and collectors were clearly identified with a phone number to call for information. If collectors are located in pasture, fencing or placing in a rocky area will be necessary. Apparently cows enjoy football as much as people do, and a collector works well for their purposes. Sun and/or salt air will crack plastic after a 6-month period; use durable, thick-walled containers and funnels.
Sampling: for the oil-type samplers, 2-3 coarse paper filters (coffee filters work fine) are placed in a small funnel. A 60 cc syringe is used to draw up a sample of water from below the oil layer in the container. Oil is wiped off the outside of the syringe, then the sample is gently ejected into the funnel containing the filters and allowed to drain into the glass sample bottle. The filters catch any residual oil in the water sample so that there is a clean sample for analysis, and the filtering process is fast enough that no fractionation occurs. For collectors in Puerto Rico, 5-gallon rectangular carboys with screw caps were used. Holes were drilled in the screw caps and funnels glued into the caps. An extra cap for sampling contained a spigot. For sampling, the funnel cap was unscrewed and replaced with a spigot cap, the carboy was turned on its side so that the mineral oil rose to the top of the sample, and a sample was withdrawn through the spigot.
Evaporation caution: At the start of our study, we used a 3-4 mm layer of oil in the collectors. Controls showed that evaporation occurred from the collectors, however, the sample composition stayed close to the meteoric water line. Therefore, evaporation was not readily apparent from the sample isotope values, but the samples did not represent rainfall isotopic composition. This process corresponds to evaporation at >90% humidity, and would likely occur with any rain collector design susceptible to evaporation. The IAEA's sampling protocol recommends that any precipitation isotope sampling interval longer than one day have a collector design that prevents evaporation.
4. 30-gallon can collection device: In a long-term research project where large samples are needed to analyze multiple isotopes in precipitation, 30-gallon (113-liter) plastic trash cans with dome lids are used for sample collection. The dome lid of the can is inverted and used as a funnel, and precipitation is collected in 20-liter collapsible carboys placed inside the can. The samplers are located in a small clearing in a forested area at 1074 m, and the samples are collected approximately every 6 weeks.
· An approximately 2-cm square hole is cut in the top center of the lid, with the lid placed right side up (convex upward).
· A plastic funnel about 7.6 cm (3 inches) in diameter with a graduated-width stem is attached over the hole with two ~3 mm diameter holes drilled in the funnel, matching pairs of holes drilled in the lid on either side of the square hole, and cable ties securing the funnel to the lid. Glass wool is placed inside the funnel to filter debris.
· Wide mesh (~1 cm) plastic netting is secured across the under side of the lid to prevent large debris from entering the finished collector.
· The 20-liter carboy is placed in the bottom of the can in its “collapsed” condition. The screw cap has a barbed nipple to which is attached approximately 4 feet of R-3603 Tygon tubing with appropriate inner diameter to fit both the nipple and funnel.
· After making a wide knot in the tubing, creating a trap that will hold a small amount of water and prevent evaporation of sample in the carboy, the other end is securely attached to the bottom of the funnel.
· Care is taken in placing the carboy with tubing inside the trash can so that the tubing is unobstructed and not crimped.
· The lid is placed upside-down on the trash can (concave upward) and secured to the can handles with plastic-coated wire (so that lid can be removed easily for sampling).
· To keep the can upright, it may be placed in a frame, or sand or gravel (at least 100 lb. (45 kg)) may be added to the bottom of the can.
· Sample collection is simply exchanging the full carboys for empty ones. The glass wool must be changed every six months to prevent excessive accumulation of algae and debris. The tubing is changed if algae or mold growth is a problem.
· Wide surface area collects large volume of water needed for analysis of multiple isotopes.
· Wide surface area also makes it possible to collect snow.
· Tubing can soften and crimp in hot weather, preventing precipitation from accumulating in the carboy. Using tubing with a wall thickness of 1/16 inch seems to help.
References describing collectors for rainfall isotopes:
Adams, A.I., Goff, Fraser, and Counce, D., 1995, Chemical and isotopic variations of precipitation in the Los Alamos Region, New Mexico, Los Alamos National Laboratory Report LA-12895-MS, 35 p.
Adar, E., Levin, M., and Barzilai, A., 1980, Development of a self-sealing rain sampler for arid zones, Water Resources Research, v. 16, no. 3, p. 592-596.
Adar, E. and Long, A., 1987, Oxygen-18 and deuterium distribution in rainfall, runoff and groundwater in a small semi-arid basin: the Aravaipa Valley in the Sonora Desert, Arizona, IAEA Report IAEA-SM-299/135.
Friedman, I., Smith, G.I., Gleason, J.D., Warden, A., and Harris, J., 1992, Stable isotope composition of waters in southeastern California, 1. Modern precipitation, J. Geophysical Research, v. 97, no. D5, p. 5795-5812.
Ingraham, N.L., Lyles, B.F., Jacobson, R.L., and Hess, J.W., 1991, Stable isotopic study of precipitation and spring discharge in southern Nevada, J. Hydrology, 125, p. 243-258.
Kazahaya, K. and Yasuhara, M., 1994, A hydrogen isotopic study of spring waters in Mt. Yatsugatake, Japan, application to groundwater recharge and flow processes, J. Japanese Association of Hydrological Sciences, 24(2), p. 107-119.
Scholl, M.A., Ingebritsen, S.E., Janik, C.J., and Kauahikaua, J.P., 1996, Use of precipitation and groundwater isotopes to interpret regional hydrology on a tropical volcanic island: Kilauea volcano area, Hawaii, Water Resources Research, 32 (12), 3525-3537.
Scholl, M.A., Gingerich, S.B., and Tribble, G.W., 2002, The influence of microclimates and fog on stable isotope signatures used in interpretation of regional hydrology: East Maui, Hawaii, J. Hydrology, 264, 170-184.
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U.S. Department of the Interior, U.S. Geological Survey, Reston, VA, USA