Jeq50458 1863..1872

This report evaluates a vacuum-assisted walled percolation sampler preconditioned in soil, and examines the dynamic response of leachate solutes. The 20-cm walled percolation sampler extracted soil water under continuous tension via a ceramic cup collector embedded in a silica flour layer, whose upper surface interfaced with field soil. In the laboratory, alternating solutions with high and low NO3–N (232 or 3.6 mg L), molybdate-reactive P (MRP) (1.75 or 0.0 mg L), K (568 or 3.6 mg L), and Br (9.6 or 0.0 mg L) concentrations were delivered directly to the (i) sampler ceramic cup; (ii) silica flour bed surface, or (iii) 12-mm soil layer placed over the silica flour bed. For alternating input solutions delivered to the silica-flour bed surface, (i) solute breakthrough (95% equivalency) occurred in 4 pore volumes and was the same for both the high and low concentration input phases of the application, and (ii) concentrations of NO3–N, Br , and MRP in cumulative extracted water volumes were within 5% of those in corresponding input volumes. Alternating nutrient loads from high to low levels in the fixed flow rate input waters caused excess MRP (1.6 times that in the high concentration MRP solution) to leach from the calcareous soil. The dynamic character of P transport in K-fertilized soils deserves further study and may have important environmental implications. UNDERSTANDING THE DYNAMICS of soil solution chemistry is a prerequisite for redressing many environmental problems, but our ability to sample soil leachate is limited. Dynamic soil solution phenomena have not been investigated thoroughly because these processes are time-sensitive and most soil solution samples are collected manually, which limits the number and frequency of samples obtained (Lentz, 2006). Porous cups or plates installed alone in the soil under continuous suction extract soil water, but may not collect all macropore water (Wilson et al., 1995) and the rate of collection may be substantially different from the soil water percolation rate (Cochran et al., 1970; Van der Ploeg and Beese, 1977). These difficulties can be avoided by deploying the porous extractor in a walled percolation sampler (Duke and Haise, 1973) because the sidewall ensures that percolating macropore water cannot bypass the extraction device and collected percolation volumes are often less sensitive to suction applied to the bottom of the walled sampler (Corey et al., 1982; Montgomery et al., 1987). Water flux and percolation volume measurement using walled samplers with ceramic extractors have been studied (Corey et al., 1982; Lentz andKincaid, 2003), but the dynamic response of these systems to changes in solute concentration and associated effects on collected sample water quality is less known. A number of researchers have investigated the effect of porous cups on extracted water chemistry. Solute sorption by porous ceramic cup samplers may alter the response of sample solute concentration to changes in drainage water chemistry, although reports in the literature are contradictory (Litaor, 1988; Grossmann and Udluft, 1991). Furthermore, since many investigations of ceramic cup solute effects were conducted using new unwashed or washed cups, but not ones preconditioned in soil environments, the reports give an incomplete understanding of cup behavior in the field (Grossmann and Udluft, 1991). Nagpal (1982), Hansen and Harris (1975), Bottcher et al. (1984), and Grover and Lamborn (1970) studied PO4–P adsorbed by new washed or unwashed porous ceramic cup samplers in the laboratory by passing an aqueous solution of known concentration through the cup. All reported that ceramic cups adsorbed PO4–P. The P adsorption by the ceramic cups was (i) greater following nontension periods between extraction events (Nagpal, 1982) and (ii) less for ceramic cups having low air entry pressures (50 kPa) relative to higher air entry pressures (200 kPa) (Bottcher et al., 1984). However, when ceramic cups were compared with hollow fiber or fritted glass samplers installed in soil profiles, solutions collected from all samplers over 5or 28-wk periods contained equivalent PO4–P concentrations (Levin and Jackson, 1977; Silkworth and Grigal, 1981). The amount of NO3–N adsorbed by porous ceramic cups placed in solutions was minimal (Wagner, 1962; Hansen and Harris, 1975; Nagpal, 1982; Poss et al., 1995). Laboratory studies indicate possible desorption and/or adsorptionofCa andMg fromceramic cups (Rasmussen et al., 1986; Peters and Healy, 1988); however, when ceramic samplers were installed in field soils and compared with fritted glass or polytetrafluroethene (PTFE) samplers or zero-tension soil water samplers with polyester mesh membranes, no consistent ceramic cup effects on leachate cation concentrations were observed (Levin and Jackson, 1977; Silkworth and Grigal, 1981; Rasmussen et al., 1986; Hendershot and Courchesne, 1991; Beier and Hansen, 1992). Both laboratory and field studies indicate minimal or no alteration of solution Na concentrations when transmitted through ceramic cup samplers U.S. Dep. of Agriculture, Agricultural Research Service, Northwest Irrigation and Soils Research Laboratory, 3793 N 3600 E, Kimberly, ID 83341. Mention of trademarks, proprietary products, or vendors does not constitute a guarantee or warranty of the product by the USDA-Agricultural Research Service and does not imply its approval to the exclusion of other products or vendors that may also be suitable. Received 15 Dec. 2005. *Corresponding author (lentz@ nwisrl.ars.usda.gov). Published in J. Environ. Qual. 35:1863–1872 (2006). Technical Reports: Ground Water Quality doi:10.2134/jeq2005.0458 a ASA, CSSA, SSSA 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: CF-soil, cup-flour-soil sampler configuration; CPV, total pore volume of sampler configuration; MRP, molybdate-reactive P; PV, pore volumes extracted; SD, standard deviation; STAS, sequential tension autosampler. R e p ro d u c e d fr o m J o u rn a l o f E n v ir o n m e n ta l Q u a lit y . P u b lis h e d b y A S A , C S S A , a n d S S S A . A ll c o p y ri g h ts re s e rv e d . 1863 Published online September 13, 2006