I've been going over basic Forensic Isotope Analysis to get some idea what the conclusions about the geographic histories of the Bear Brook Four are all about.
In a nut shell, the the tap water in different cities and towns have different ratios of two isotopes of oxygen and these ratios which are determined by local geology and distance from the ocean, will remain pretty much the same in any given water system, from year to year. This is called the O2 16/18 ratio.
The O2 16/18 ratio in human tissue reflects the ratio is the water they drink, normally from the tap. Most tissues regenerates gradually over time so the ratios would gradually change if a subject moved and a reading would be worthless. Two types of human tissue that does not change over time are tooth enamel,which is fixed during the first few years of life and hair, which will will reflect the O2 16/18 ratio of the local water at the time it emerges from the follicle and remain fixed at that point as the hair grows. Since hair grows approx 6" a year, a 12" hair would provide a two year linear record of the isotope environments the subject was exposed to.
Apparently the maps that we have seen are based on isotope studies of the local water in 65 different US cities. I don't know if the same information is available for Canada.
If a subject drank mostly Bottled Water, not of local origin, ate other than a "typical" mix of food, or drank well water or water from isolated sources, their ratio may not correlate to the area they live. Overall, it is pretty accurate in narrowing down where someone spent their infancy and, depending on hair length, where they spent the last months or years of their lives.
One obvious limitation of this method is that some O2 16/18 ratios are fairly common. The woman and the two related children appear to have spent their infancies and the last year or so in an area that had a ratio consistent with the Manchester NH area but there are many places they could have lived with a fairly dense population.
The biological child of Rasmussen/Evans came from a far more limited area with a much lower population. We do know that that child only moved to an area with a O2 16/18 ratio like that of Manchester, in the 3 to 4 months prior to her death.
There are other elements that have isotope ratios that can be used forensically but the science is less developed. Some of these may have been used but they have not been publicized.
Attempting to locate missing mothers with children from the geographic limits of their O2 16/18 ratios is pretty much a needle in a haystack situation with one haystack being especially big. What we can probably assume is that the middle child spent her entire life in one area until she moved to NH in the months before her death and the other every likely did as well. (It seems unlikely that a nomadic existence would be limited to one O2 ratio area).
I do not believe Rasmussen/Evans would have transported four bodies a long distance so I think it is very likely that all of them were living together in the Manchester area in the 3 or 4 month period before their deaths.
Texas however, where Rasmussen/Evans is believed to have lived before he moved to NH is not one of them.
