WUR Colloquium Nick Gorski

Datum publicatie 04-06-2010

Datum: Dinsdag  8 June  2010  

Tijd: 11.00  - 11.45 uur 

Plaats: Zaal  3,  Atlas Gebouw,  Droevendaalsesteeg 4, Wageningen 

Begeleider: Sjoerd van der Zee

Abstract
Unsaturated zone processes remove solutes and pollutants from infiltrating water and thus play a vital role in protecting groundwater resources from contamination. However, natural soils and soils with spatial variability often exhibit preferential flow (PF) of water and physical non-equilibrium (PNE) in solute transport. This can result in quicker solute breakthrough times, higher than expected cumulative leaching and incomplete degradation or retention of harmful reactive compounds such as pesticides before reaching the groundwater table. This paper utilizes data from a previous transient flow Cl^- tracer experiment with a multi-compartment lysimeter of 300 sampling cells (0.75m² area) on an undisturbed sandy soil column of 0.50-0.55m depth under laboratory conditions. A 0.98M pulse application of CaCl₂ tracer was applied to the column and allowed to leach for 57 days with water applied in 20mm increments every Monday, Wednesday and Friday. The spatio-temporal variability of tracer leaching in the individual sample cells were evaluated using temporal moment analysis and compared to leaching over the full lysimeter. Sampling cells were then amalgamated into groups with similar leaching properties and modeled as a series of non-interacting parallel columns in HYDRUS-1D. Input of water and tracer for each sample cell group or column was multiplied by a productivity ratio (PR) that conceptually represented an increase or decrease in the cell group’s hypothetical capture zone based on the cumulative leached fraction of the cell group in the column. Modeled column breakthrough curves (BTCs) were calibrated using dispersivity and all other soil hydraulic parameters were similar across all columns. Results showed that the model better predicted tracer leaching from the fast and more productive columns whereas it was less successful in predicting BTCs from the less productive columns, likely due to the lack of a redistribution mechanism. Fast-leaching, productive columns were then re-modeled using a simple reactive compound to show the effects of heterogeneous flow on solute breakthrough time and amount. Temporal moment analysis of individual cell and grouped BTCs under varying dispersivities suggest that the time-to-peak, variance and skewness of a BTC for the low productivity cells and groups can be used to estimate the time and location of lateral redistribution of water and solute. The modeling methodology employed is a useful, easily-parameterized tool that is suitable for predicting worst-case scenarios and risk assessments of pesticide and solute breakthrough in heterogeneous, non-structured soils. The findings underscore the fact that significant insight on heterogeneous flow processes in the unsaturated zone remain to be discovered from large, high resolution multi-compartment sampler (MCS) datasets.

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