Frequently Asked Questions

1. If I run LandSim twice, the results aren’t exactly the same, particularly at the extreme percentiles.
The output results from a LandSim assessment shows the results for all iterations in a single simulation. Results for subsequent simulations may be slightly different, particularly at extremely high or low percentiles. This is due to the random number generator “seed value” used in model runs. To give some indication of the extent of this effect, you can view confidence limits (through File – View – Confidence Limits on each graph menu). Individual confidence limits viewed (for example, the 99th and the 1st percentile, or the 95th and the 5th percentile) are selected using File – Options – Confidence Limits on each graph menu.

Using a large number of iterations and not concerning yourself with absolute extreme values will minimise any problems associated with the different random number seeds used in LandSim.

2.The leakage rates output by LandSim don’t match those I calculate by hand or in a spreadsheet.

Leakage is limited to the total liquid input to the site (you can’t have more going out than is coming in). The situation usually occurs when a fixed head is being modelled. You should compare the leakage results with total infiltration (infiltration in mm/year multiplied by site top area in Ha and adjusted to l/d). If the two values are the same, you can reduce the fixed head if it is unrealistically high, or increase the infiltration value – but be aware this may result in non-conservative results if you are modelling a declining source.

If you are modelling a membrane or composite liner system, make sure you are calculating the leakage using the same method as LandSim. The software uses the Giroud & Bonaparte equations to calculate leakage through an individual defect (hole, pinhole or tear) before multiplying by the number of defects per Ha and the site basal area. You should also check that the value you are using for the constants specified in the published equations (which relate to the closeness of contact between the membrane and the underlying layer) match those used in LandSim. See the Theory section in the LandSim manual for more information.

3. Why can’t I enter retardation or biodegradation parameters ?

You have to specify that the model is to run in retarded mode, or including biodegradation. This is done under the ‘simulation – options’ menu, and your choice has to be specified separately for each pathway.

4. What is the difference between the monitoring point and the compliance point ?

You can position a single Compliance Point at any point downgradient of all Phases of the landfill. Results for this Compliance Point show the combined impact on groundwater from all Phases. Each Phase has a monitoring point fixed to the downgradient edge. This monitoring point is used to view groundwater impact from the Phase it is attached to and will not ‘see’ contamination arising from any other Phase. Thus, if an impact is indicated at the Compliance Point, viewing the results for each monitoring point will enable you to examine the relative contributions from each Phase.

5. How do I model a BES, DAC or GCL liner ?

Bentonite Enriched Soil, Dense Asphaltic Clay or Geosynthetic Clay Liners can all be modelled in LandSim simply by setting thickness and hydraulic conductivity values appropriately.

6. LandSim seems to have calculated an instantaneous travel time – why is this ?

There are two possibilities.

It may be that the unsaturated zone thickness has been set to a very small value, and it therefore takes less that a year for leachate to reach the water table. The shortest time increment used in LandSim is one year, and results less than this may appear to be instantaneous, particularly if some iterations realised longer travel times.

Alternatively, in the rare cases where a membrane or composite liner is specified over a very small cell area (‹ 1Ha), then some iterations may realise zero defects, and hence no leakage. If there is no leakage, then the flow velocity is calculated as zero, and the travel time (distance divided by velocity) is infinite. It is unlikely that this will occur for all iterations in a simulation, but it will affect the results at the extreme percentiles. A possible solution is to increase the minimum number of defects specified.

7. I’m trying to examine the concentration of contaminants at the compliance point, but the graph is blank. What has gone wrong ?

Sometimes every iteration will realise the same result, and this cannot be plotted on a probabilistic graph. LandSim will display a message informing you that the result is a single value, and will not plot. It will also tell you what the value is.

If you haven’t got this message, then there are a number of reasons for a blank graph:

• the graph scale is inappropriate. By double-clicking on the concentration (x) axis, you will be able to change the minimum and/or maximum plotted values, or plot the results on a log scale. The results should now be visible.
• the time slice you are looking at is less than the contaminant travel time (retarded). Since the contaminant has not yet reached the receptor, the concentration is zero (assuming no background), and the graph will be blank. the contaminant has degraded before it reaches the receptor. If you are modelling a degrading species, compare the contaminant half life with the retarded travel time to see if this is a possibility.
• the cation exchange capacity has not been exhausted. If you are modelling ammoniacal nitrogen in unretarded mode with a declining source, it is possible that there is sufficient CEC to take up all the ammoniacal nitrogen. Check the CEC Exhaustion Time under the ‘time for’ in the results menu.

In all cases, you can check the statistical results to see numerical values.

8. Does the Total Geosphere travel time include the time for the contaminant to travel through the liner ?

No. It is measured from the base of the liner to the receptor.