Source Water Protection - Public Forum - How timie-of-travel ...

Topic: How timie-of-travel is arrived at

Replies: 5
Last Post: Apr 7, 2009 11:35 AM By: Tim Cumming

Tim Cumming Staff Member Posts: 23 Registered: Jul 13, 2006	Reply To This How timie-of-travel is arrived at Posted: Nov 4, 2008 10:21 AM
	Brian: Municipalities in our region are now helping expand the delivery area for stewardship funding by adopting two-year time-of-travel lines for the sole purposes of the Ontario Drinking Water Source Protection Program. Could you explain how those lines are arrived at? Thanks, Tim

Brian Luinstra
Staff Member
Posts: 2
Registered:
Jul 29, 2006

Reply To This

Time of Travel Well capture zones
Posted: Nov 17, 2008 3:00 PM

in response to: Tim Cumming

Time of Travel (TOT) capture zones are used to determine where water that is being pumped by a well may have originated from. In order to do this we follow these steps

1. We set up a groundwater model (the computer kind - no clay involved) to simulate how groundwater is flowing in the aquifer that the well is exploiting. In doing this, we account for the hydrogeology of the aquifer (things like porosity, hydraulic conductivity, etc), the wells tapping the aquifer and what we know of the water table (or piezo-metric surface) in the aquifer. This model is then calibrated by comparing it to known water levels, usually derived from water well records.

2. We then run the model backwards in time. Sounds like something out of Star Trek but this is what we do. We insert a number of neutral buoyancy (i.e. same physical characteristic as water) particles in the well and run it backwards. In order to develop a 2 year TOT capture zone, for example we would run it for 2 years back in time and see where the particles go. We call this "Reverse Particle Tracking". Run the model a few times for a 2 year period and when you are done we have a whole lot of dots which represent where the particles ended up in their trip back through time, over which we then draw a line, projected on the ground surface to make a 2 year TOT capture zone.

We do the same for the other times involved, in this case 5 years, and 25 years.

Make sense?

Brian Luinstra
Staff Member
Posts: 2
Registered:
Jul 29, 2006

How accurate are TOT Capture Zones?
Posted: Nov 17, 2008 3:37 PM

in response to: Brian Luinstra

This is a common question so I thought I would address this in this forum.

There are a number of sources of error associated with the development of TOT capture zones, namely:

1. The quality of the data used to develop the groundwater model.

In SW Ontario, the primary data source which we have to rely on are water well records, which are submitted by well drillers after any well is drilled or has significant alterations. These records are often lacking in their geological information and more commonly are located incorrectly. The biggest problem in this regard is that there simply aren't enough wells in the area.

Water levels used to calibrate the models also come from these records. They can be a bit problematic as the water levels are recorded when the well was drilled, and water levels fluctuate over time, especially seasonally.

2. The models themselves. I often muse that "models are only slightly worse then the data that goes into them". All joking aside, there is some truth in that statement. In the end, it is a model that was used and all models have inaccuracies in how they calculate them. No model can completely predict the complexities of the natural system.

Geologists, in general, are accepting of these types of error as it is inherent to the study of the natural systems. Unlike engineering, where systems are designed with exact tolerances, geology is a descriptive science. Geologists rarely get a complete data set (as most of the evidence is buried) and don't get to run experiments on the systems they are observing. As a result, we must make interpretations to do our work. Many a Geologist has made really solid interpretations based on the best available information only to have someone drill a hole and completely change the understanding of the geology of an area. As a result of this, Geologists have become very conservative in how they interpret information.

There is some good news, however. Ontario is is a world leader in hydrogeology. In fact, the first hydrogeological modeling in the world was completed in Ontario, and our research and teaching institutions are renowned world-wide for the quality of their work.

In our region, we have contracted some for the best and most well-respected modelers in the world, and striven to gather the best quality data we could find for incorporation into our models. This means that our Well Head Protection Areas are a reflection of the best available science.

One could ask - what more could we do?

Tim Cumming Staff Member Posts: 23 Registered: Jul 13, 2006	Re:Time of Travel Well capture zones Posted: Nov 19, 2008 1:16 PM in response to: Brian Luinstra
	Why doesn't the WHPA in all areas have a 25-year capture zone?

Tim Cumming Staff Member Posts: 23 Registered: Jul 13, 2006	Re:Time of Travel Well capture zones Posted: Nov 19, 2008 1:17 PM in response to: Brian Luinstra
	How will Drinking Water Source Protection use Well Head Protection Areas (WHPAs) to determine vulnerability?

Tim Cumming Staff Member Posts: 23 Registered: Jul 13, 2006	Reply To This RE:How time-of-travel is arrived at Posted: Apr 7, 2009 11:35 AM in response to: Tim Cumming
	We have now produced a local fact sheet about the rationale behind the 100-metre well head lines and how two-year time-of-travel lines are arrived at. Let me know if you would like a copy. Tim. 1-888-286-2610.