Simulation of picloram, atrazine, and simazine leaching through two New Zealand soils and into groundwater using HYDRUS-2D

• Published in: Journal of contaminant hydrology Vol. 44; no. 1; pp. 19 - 46
• Main Authors: Pang, Liping, Close, Murray E, Watt, James P.C, Vincent, Keith W
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 01-06-2000; Elsevier Science
• Subjects: Atrazine; bromides; Degradation; Earth sciences; Earth, ocean, space; Engineering and environment geology. Geothermics; Exact sciences and technology; Groundwater; Hydrogeology; Hydrology. Hydrogeology; HYDRUS-2D; New Zealand; New Zealand, Hawkes Bay; Picloram; Pollution, environment geology; Simazine; Soils; Sorption; Surficial geology; Unsaturated zone; New Zealand; North Island; Simazine; Sorption; Degradation; Unsaturated zone; HYDRUS-2D; Picloram; Groundwater; Atrazine; bromides; Australasia; ground water; water table; degradation; digital simulation; leaching; silt; transport; pollution; halides; sorption; infiltration; unsaturated zone; gravel; sand; aquifers; atrazine; pesticides; water content; soils; boundary conditions; loam; organic carbon
• ISSN: 0169-7722; 1873-6009
• DOI: 10.1016/S0169-7722(00)00091-7

Two 15 m×15 m field plots, a Te Awa silt loam and a Twyford fine sandy loam, located in Hawkes Bay, New Zealand, were applied with bromide, picloram, atrazine, and simazine. The Te Awa subsoil was a heterogeneous coarse sand and sandy gravel, and the Twyford subsoil was a more homogenous fine sandy loam. The underlying aquifers were composed of alluvial gravels at both sites with the water tables generally between 4–5 m below ground level. The sites were monitored for 2.2–3.5 years at approximately monthly intervals using suction cups in the unsaturated zone and monitoring wells in groundwater. HYDRUS-2D was used to simulate water movement and solute transport in soil and groundwater in a domain with a depth of 10 m and length of 68 m, including a 4.5-m unsaturated zone. The model simulated well the general trend of field observations for soil water content (
θ) and potential (
ψ
s), and the values matched better for the soils with less heterogeneity. For the soils with significant surface cracks, the simulated
θ values were overestimated. On the other hand, for the soil layer perching on top of a less permeable layer, the simulated
θ values were underestimated.
Simulated pesticide concentrations using the “best available literature values” (BALVs) of organic carbon distribution coefficient (
K
oc) and half-life (
T
1/2) were generally lower than those observed. At early times in the trails, most simulations using BALVs were still within the same order of magnitude as observed concentrations for the shallow depths. However, at greater depths and later times, there were major differences between observed and simulated concentrations. The model was then calibrated for
K
oc and
T
1/2 values using observed data with an aid of the PEST optimisation package. Despite higher organic contents found in the topsoil, optimised
K
oc values for pesticides were consistently lower for the topsoil than for the subsoil, and were also lower than the BALVs except for picloram, possibly as a result of preferential flow in the topsoil. Compared to the BALVs, picloram was much more persistent but slightly more sorptive in both soil types, and atrazine and simazine were more persistent in the Te Awa soils but less persistent in the Twyford soils. The optimised
K
oc values for all three pesticides were generally greater, or close to, the BALVs in the subsoil of both sites.
HYDRUS-2D provided a reasonably good link for pesticide transport in both the unsaturated zone and groundwater. Simulated pesticide concentrations in groundwater using optimised values were generally similar to the observed values. Both observed and simulated bromide and pesticide concentrations indicated that solutes leached more quickly through the soils that were coarser and more heterogeneous, but were more diluted in the groundwater system that was more heterogeneous, conductive, and dispersive. Significant levels of picloram were found in groundwater 22 and 53 m down-gradient of the Twyford and Te Awa plot, respectively. Both simulated and observed atrazine and simazine concentrations in groundwater were less than detection limits.