Importance of the nugget effect in variography on modeling zinc leaching from a contaminated site using simulated annealing

Author(s)

Thilo Hofmann, Andreas Darsow, Maria-Theresia Schafmeister

Abstract

The spatial heterogeneity of measured data from contaminated sites is a serious problem with regard to

uncertainty and risk assessment. Stochastic simulation permits the generation of several realizations of

the spatial data distribution without aiming to minimise local error variance (as in kriging, for example),

but retaining important statistics such as the histogram, the semivariogram, and the measured data. The

nugget-to-sill ratio will, however, influence spatial data interpolation based on geostatistical models. The

nugget effect is generally unknown to the investigator and influenced by the sampling density and sampling

grid, as well as by complex geological settings, analytical errors, or an unknown small-scale sitespecific

contaminant input. Using simulated annealing to generate 30 randomly combined input data

realizations and a simple transport model, we can show that the modeled zinc discharge is up to 70%

higher if the nugget-to-sill ratio is varied by a factor of two times. The variance of zinc discharge was

influenced by the nugget-to-sill ratio by a factor of 10 times. By reducing the number of drill holes to

one half from the data set at random we have shown that the variance of the reduced data set did not

provide a reliable prediction of the data heterogeneity for the complete data set, and the mean zinc discharge

was up to 170% higher when only half of the drill holes were considered. Uncertainty analysis

based on equal-probability realizations should further be founded by additional variation of nugget-tosill

ratio within a reasonable spectrum.

Organisation(s)

External organisation(s)

University of Greifswald

Journal

Journal of Hydrology

Volume

389

Pages

78-89

No. of pages

12

ISSN

0022-1694

DOI

https://doi.org/10.1016/j.jhydrol.2010.05.024

Publication date

2010

Peer reviewed

Yes

Austrian Fields of Science 2012

105303 Hydrogeology, 104023 Environmental chemistry, 105105 Geochemistry, 105904 Environmental research

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