2013; A Wet Summer to Study Drought

Hello!

My last post was an introduction to the Jamestown Colony, what I do when I am out there, and some pictures. This post, I’d like to talk a bit about the progress I have made so far this summer and what major questions I have remaining.

First, a reminder of what I am doing: I am studying the water quality of the early Jamestown colony with the goal of evaluating claims that the salinity of the drinking water was to blame for a high disease rate. I am especially focusing in on the salinity communication between the James River and the aquifer in which the colonists’s wells were located. The background can be found in my first post.

What I Actually Do 

To start, this summer has been wet. Really wet. I am trying to draw conclusions about the effects of the worst drought in 700 years on the groundwater system. Sigh.

The effects on the James River have been significant. Generally, the James River’s discharge falls in the summer, and is accompanied by a spike in hydraulic conductivity, which is our proxy for salinity. Less water down river, more salt water from the ocean. Very generally.  This summer is different, as we have yet to see a rise in the James River even though the summer is over half over. The graph below is the conductivity of the water from 2007 to present, recorded from a NOAA bouy in the James River . For perspective, bottled water would be about 200 uS/cm, and seawater is over 50,000 uS/cm. So you can see that the summer usually gets to about 1/3  as salty as seawater before dropping in the spring. Usually, just after June 1st of every year there is a spike in water conductivity, but not this year. (Note: the buoy sensor was out of order in 2009 for a bit). Just why this summer is different is something I am working on. A severe drought, we think, would have the opposite effect of this year.

2007-2013 Spec Cond

What the James River does is all well and good, but for our purposes we need to know if and how river water makes it into the groundwater. There are 2 main ways I am doing this. One is to look at the elevations of the groundwater level  in our 19 groundwater test wells through time. Water, to put it very simply, flows down gradients in the water table (for a surficial aquifer like ours is). So if the river is higher than the groundwater, the river will flow in. If one side of the aquifer has a higher water table elevation than another, water will flow downhill. So one important thing I am measuring is the elevation of the water table. *Note: I am actually measuring the hydraulic head, but I’m keeping this simple. Below is a contour map of the water elevation, with the flow direction drawn on with arrows. For this particular time, water is flowing from the swamp to the river. But this does vary across time. 7-10-12-pjg

How quickly water flows in any groundwater system is dependent upon the properties of the aquifer. Clay, for instance, will not let as much water through as will gravel. So I have conducted many tests on the aquifer this summer to find out what the range of groundwater velocities is and if there are vertical variations in the sediment here that may create preferential flow paths  beyond what you might expect. So this has involved pouring through the well log descriptions that were made by previous geology researchers and trying to piece them together into cross sections and tracking maps.

Tidal oscillations are important here, as there are two high and two low tides a day, which are communicated into the aquifer. The dataloggers that we have in some wells pick up on the extent to which the high or low tide is communicated. Equations have been developed that allow you to look at how the oscillation attenuates to determine additional properties of the aquifer. We have found some extremely puzzling tidal amplitudes in certain areas of our aquifer that are completely contrary to our expectations, so I am very much in the process of sorting this stuff out.  Below is one day’s worth of data logger data. Notice how the amplitude of the tidal signal is drastically different among wells (usually according to how far from the river they are, but not always).

May 28 TIdal Oscillations

The light teal is the James River, and the other lines represent various wells.

 

So far I’ve talked about how I’m trying to figure out how water flows in the aquifer. But I am also thinking about how saline water is distributed and how long it takes for a spike in the James River to be felt in the groundwater here. Because groundwater moves so slowly, there is a significant delay in how dissolved ions are transported. In this system, we’d expect it to take months to years for changes to be felt.

Here are my current questions that will keep me occupied for pretty much the rest of the summer:

1) What can tidal oscillations tell us about the properties of the aquifer, and why do some strange anomalies in the tidal oscillations exist?

2) What effect does tidal oscillation have on groundwater flow velocity in the long term?

3) How much lag time exists between conductivity spikes in certain wells and what does that suggest about the ways saline water is entering the aquifer?

4) How has the physical environment changed in the past 400 years, and what effects would it have on the hydrology?

The Process of Research

I have learned that, so far, everything I have set out to accomplish is approximately 17x more difficult and takes 31x longer than I thought. There are days when, seemingly, my only accomplishment has been to learn something new about Excel (after several hours of trial and error, mostly error). But there are also those rare days when the work you have been doing has set you up to make a substantiated claim about something you can say you reasonably know. Those times are more rare than I imagined, but I have a new-found appreciation for every new study that I read and  how difficult it is to reach conclusions that have a high degree of certainty.

My subject has its own set of challenges. Unlike structural geology, for example, the groundwater system changes even as I measure it. This system is so dynamic and has so many variables that it defies simple understanding. That is why, undoubtedly, I will be collecting data right up until I publish my thesis. What conclusions can you draw about something that continually changes? I do not have the luxury of a laboratory that would allow me to set the system to “700 year drought conditions” or to reverse the meander of the James River. I hope to try to approximate this with numerical modeling later on, but modeling has its own set of limitations.

I take solace in the fact that I am not alone with my confusion. I’ll leave you with a quote (properly cited!)  I stumbled across while reading an article.

“While ground-water hydrology in theory is an exact science, in practice it is far from that because the underground environment defies precise characterization…. Perhaps the very fact that underground flow systems defy precise characterization makes them so challenging, like the mysterious person in our life whose depths we try to probe and whose secrets we try to unveil!”
 

Bouwer, Herman. 1996. Discussion of Bouwer and Rice Slug Test Review Articles. Ground Water. v. 34, no. 1, pp. 171.