The source of elevated constituents is speculative, but... - Part 6

The paper titled "An evaluation of water quality in private drinking water wells near natural gas extraction sites in the Barnett Shale Formation" states on line 285:

Samples that exceeded the MCL for TDS, arsenic, and selenium were located an average of 1.1 km from the nearest natural gas well. Similarly, the highest values for both strontium and barium were over twice as high in areas less than 2 km from the nearest natural gas well compared to more distant gas wells.

Let's look at the graph with the green, yellow, and red dots.

 Notice that gray shading?  That's all the oil & gas wells in the area.  The Railroad Commission of Texas (RRC) has a web site devoted to the Barnett Shale.  Here is their map.

RRC

Now let's look at the number of wells in the area:

RRC

There are a little shy of 17,000 wells in the area.  That's why there is so much gray in their graphic.  There are four counties where most of these wells exist; Denton, Johnson, Tarrant, and Wise.

The UTA research found that "arsenic, selenium, strontium, barium, and TDS reached their highest concentrations in areas of active extraction in close proximity to natural gas wells."  The source of this arsenic is unknown, but the paper points the reader in the direction of the gas wells as the culprit.

Line 291: The geographic patterns in our data suggest that lowering of the water table during a drought period cannot fully explain these elevated constituent levels.

...and...

Line 293: Concentrations that exceed the MCL occur only in close proximity to natural gas wells suggesting that mechanical disturbances or localized groundwater withdrawals near natural gas wells could play a role in elevated constituent concentrations.

...and...

Line 312: It is also possible that improper handling of waste materials and faulty gas well casings could result in the introduction of these compounds into shallow groundwater

 I think we can rule that last one out due to no BTEX being found in any of the 90 wells they sampled.

So what about the arsenic?  Line 322:

Arsenic showed a significant positive correlation with TDS suggesting that it may be concurrently mobilized into groundwater with TDS during the natural gas extraction process.  Again, mechanical disturbances (high pressure fluid injection, mechanical vibration, etc.) associated with natural gas extraction activities could be the cause of elevated levels of TDS and arsenic.

Now to tie it all together.

• Line 339: Concentrations of arsenic, strontium, and selenium were significantly higher in samples from active extraction areas compared to historical data.
• Line 341: Non-active/reference area samples also showed a significant increase in arsenic compared to historical data.
• Line 342: Both active extraction and non-active/reference areas showed a significant decrease in barium concentrations from historical levels.
• Line 344: Historical TDS concentrations were not significantly different from non-active/reference area concentrations but were significantly higher than active extraction area samples.

And conclude:

Line 348: While we cannot draw definitive conclusions due to the fact that the historical data was collected under different sampling conditions, these data do provide a baseline for comparison to pre-industrial conditions which is generally lacking in studies of this nature.

From my point of view, if "these data were compared to a historical dataset from the same
aquifers prior to the expansion of natural gas extraction activities," then any impact from natural gas drilling would also show up in those wells post-1999.

The paper's general hypothesis is that "the geographic locations of elevated constituent levels in our study are consistent with the notion that mechanical disturbance of private water wells and industrial accidents (e.g. equipment failure, faulty well casings, fluid spills, etc.) are more frequent in areas where natural gas extraction is active."

If that is true, then the same wells used for the historical data should also see an increase in these constituents, in particular, arsenic. The paper is keen to explain finding no contaminants in other wells close to drilling:

While our data indicate elevated levels of potentially harmful compounds in private water wells located near natural gas wells, it is important to recognize that there were also a number of private water wells in close proximity to natural gas wells that showed no elevated constituents.

I am left with this:

Concentrations were significantly higher in active extraction areas compared to reference samples and historical samples.

That historical data they compare it with - the before and after - came about like this:

Historical data for the concentrations of target compounds (except alcohols) in private water well samples from this region were obtained to evaluate their occurrence before the expansion of natural gas extraction activities. This historical dataset is comprised of 330 private drinking water wells from the Trinity, Woodbine, and Nacatoch aquifers sampled over a ten year period (1989 – 1999) before natural gas activities began. Wells were located in the same counties that we sampled in this study.

This got me thinking.  If the arsenic is elevate due to proximity to a gas well, and "mechanical disturbances (high pressure fluid injection, mechanical vibration, etc.) associated with natural gas extraction activities could be the cause of elevated levels of TDS and arsenic," we would expect to see elevated arsenic in the wells from which the historical data was collected.

You know, kind of a goose and gander type thinking.  I chose Tarrant County to look at since there are a lot of gas wells there and it had a nice cluster of red dots indicating arsenic above the MCL.

The paper cites the Texas Water Development Board Groundwater Database Website as where they got this historical data.  So I went there, scrolled down to "Tarrant County" and went looking for a groundwater well that was in this area.  I then clicked on the "Infrequent Constituent Report." which is where they obtained the pre-1999 data.  I then looked for a water well in the vicinity of the red dots.

Using the "Wells in TWBD Groundwater Database-Texas website, I found a few wells that had post-1999 data.  325103, 325104, and 325102.

Source

I then went to the Railroad Commission's website and searched to see if there were any gas wells in the area.

Source

So looking at groundwater well 3205103 you can see that it is less than 1 km from gas wells.

I then went back to the "Infrequent Constituent Report" and looked at the most recent analysis on arsenic:

Okay, that's just one well.  Look at the green dots in the graphic!  Remember the authors tell us:

...it is important to recognize that there were also a number of private water wells in close proximity to natural gas wells that showed no elevated constituents.

True that, but when I look at the historical data post-1999, I find nothing to support the hypothesis "suggesting that mechanical disturbances or localized groundwater withdrawals near natural gas wells could play a role in elevated constituent concentrations."

So what gives?  Why did they find arsenic above the MCL in 29 of 90 wells they sampled?


Next post: Part 7

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