Abbott, M. D., Lini, A., Bierman, P. R., and Wright, S. F., Stable

isotopic measurements (H, O) constrain groundwater recharge mechanisms in

upland fractured bedrock aquifer, Vermont, USA

Anders Noren

3/23/99

The authors used measurements of d18O and dD in spatially and

temporally distributed samples of precipitation, snowpack, and groundwater

to assess the hydrologic regime in a catchment on the western side of Mt.

Mansfield. They analyzed samples of groundwater and precipitation

collected weekly, and determined correlation (or lack thereof) with

elevation and time of year. These trends are used to determine the amount

of mixing of groundwater with precipitation, the temporal and spatial

response of groundwater to precipitation events, and ultimately to

delineate the recharge zones for a well in the lower part of the

catchment.

In general, the science behind this research is sound. The data

clearly support the conclusions drawn by the authors, although I still

dont understand the importance of the dD measurements. A couple of their

assumptions seem somewhat sketchy. Because we are not told to which

journal(s) the authors will submit this paper, my evaluation is limited by

a not knowing the intended audience. However, I have two major concerns

that should be addressed before publication in any journal. First, the

significance of this study needs to be elaborated and clarified. It should

be presented in the introduction and reiterated in the discussion and

conclusion. I read only the slightest hint of "why this is important" in

the first paragraph of the paper; the rest of the paper seemed too raw in

its scientific endeavor to be of interest to more than a handful of

people. Second, the results and the discussion are confusing because of

organizational difficulties inherent in working with several data sets.

These sections could be clarified greatly by the use of a few subheadings

to visually separate the different data sets used in this paper.

I recommend this paper for publication after the authors address

these two major issues, as well as the comments written on the manuscript.

The submitted paper by Abbott et al uses the isotopic compositions of

Oxygen and Deuterium to better understand the path of groundwater recharge

in a Vermont watershed. Abbott et al measure the isotopic ratios of both

rainfall and snowmelt, and compare those values to the isotopic

composition of groundwater measured at 3 wells and 6 groundwater springs.

This comparison allows the authors to determine the amount of water mixing

occurring within the bedrock aquifer, and thus the residence time of the

water within the aquifer. The authors found that at lower elevations

within the basin, evapotranspiration during the summer months prevents the

infiltration of rainwater into the bedrock aquifer. This effect causes

the aquifer underneath portions of the basin below 750 m in elevation to

be recharged only seasonally, during the winter months. At elevations

above 800 m, where vegetation is sparse, recharge occurs continuously

throughout the year. Hence, even though the upper portion of the basin

comprises only 36% of the total watershed area, it contributes 58% of the

annual recharge to the aquifer.

Taken in sections, this paper is clearly written and easy to understand.

It provides a thorough explanation of the isotopic values, how they were

derived and what they mean. However, there is a missing connection

between what is being discussed in each section and how that contributes

to the larger conclusions of the study. The abstract and introduction

fail to make a clear statement about where the paper is headed, and what

exactly the result of the study is. The introduction states that the

paper will "develop a conceptual model of groundwater recharge and flow."

The following sections make no mention of groundwater recharge until page

8, and no mention of groundwater flow paths until page 10. This

organization makes it difficult to understand the contribution of the

isotopic values presented to the inferred conclusions. The Precipitation

Analysis and Meltwater Analysis sections made little sense to me, in terms

of their importance to the study, until I got to a more thorough

discussion of the groundwater results. Additionally, from both the

abstract and the conclusion, it seems that the real thrust of the paper is

the influence of seasons, vegetation and elevation on groundwater recharge

rates. I have not been able to figure out how this study contributes to a

better understanding of groundwater flow in this watershed (as was

mentioned in the introduction). A shift in emphasis and a revised

organization may help to really focus this paper on the study's results

and conclusions.

The data is impressive and appears sound. The calculations, tables, and

graphs appear to be thorough. Some further discussion of the

graphs/figures in the text would be useful. Figures 3a, 3b, 3c, and 3d

were especially confusing and I had difficulty understanding exactly what

they were meant to show. I am not sure if all of the figures are

necessary - they seemed redundant and could possibly be consolidated. The

Deuterium values are rarely mentioned. What was the purpose of the

Deuterium measurements and what exactly were the conclusions from that

data?

I feel that this paper is worthy of publication due to the evidence it

provides for the heavy influence of vegetation on groundwater recharge

rates, which is tied into elevation and seasonal variations. The paper

does however need some major revisions before being complete. An

organization and focus that will more easily guide the reader to the

conclusions of the study is required. Also a more thorough integration of

the figures into the text, and perhaps a reduction in the number of

figures, would allow the text to flow better. A better description of the

Deuterium part of the study is also missing from the paper's current form.

Manuscript:

1. What is ASL? Above sea level? You need to write that out somewhere. Is

it even necessary to say ASL - isn't that the standard?

2. What is the influence of this type of soil on the groundwater

recharge? What is the purpose of mentioning this?

3. There are only 6 precip. locations on the map - where are the other 3?

4. Where are wells 7a and 7b on the map? Are they both located at GW7?

5. It's confusing when you say that the rest of the "wells" are springs -

how can it be a well and a spring at the same time? Maybe you could say

that the rest of the sampling locations are springs.

6. RE: Figure 2. Station P7 shows data for snow samples despite the fact

that the caption says that snow samples were not collected for stations

P5, P7 and P9. Also, Figure 2 is a large figure but receives very little

attention in the text - some further explanation would help to justify its

presence.

7. I would mention the R2 values in either the text OR the figure caption.

Mentioning it in both places is redundant.

8. How does this information contribute to tracking groundwater recharge

and flow? How does it fit in to the larger picture of what you were

trying to do in this study?

9. I have the same question for this section as in #8.

10. What does that mean, "only slightly correlated"? Try to be more

specific than that. Also, what is the correlation supposed to show?

11. How will this finding contribute to tracking groundwater flow and

recharge?

12. I am not sure how figure 5 shows that - further explanation would be

helpful.

13. There is no mention of figure 8 - what happened?

14. How did you choose to set up the transitional zone between the

elevations of 750 and 800 m? Are they arbitrary values, or did you

somehow calculate these? Why isn't the transition zone between 830 and

740 m?

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Always maintain enough art in your life to keep the science from

becoming too stuffy.

Always keep enough science in your life to prevent the art from becoming

too weird.

Review of Abbott et al., 1999

Abbott et al. have measured dD and d18O in precipitation and

groundwater samples from a high-relief watershed in Vermont. Samples were

taken from several locations on a weekly basis for a year and a half,

giving them both a spatially- and temporally- significant look at the

connection between precipitation and groundwater recharge. Isotope values

in the precipitation vary widely throughout the year and according to

elevation, which means that Abbott et al. can "trace" the precipitation by

measuring isotopes in wells and springs. It turns out that

evapotranspiration plays a significant role in how much water is available

to recharge aquifers. High elevation groundwater isotope values vary the

most, because there are too few plants to cause significant

evapotranspiration of the original precipitation. Isotope values in

low-elevation groundwater during the wintertime also have a wide range

because of this same reason. However, during the warm months, the low

elevation groundwater isotope values hardly vary at all, implying that the

groundwater is well-mixed and not coming "directly" from a recharge zone.

This paper is okay, though I was left with a feeling of wanting to

know more about why this study is important. The authors could make more

of a point about why we should care where and when most of an aquifer's

recharge occurs (issues that they could discuss--pollution, snowmaking,

drought...). The presence of a town downhill from this aquifer is

mentioned, but I wasn't left with the idea that this study would make a

difference to them. I also wondered which journal this paper is heading

for (that might be in the syllabus, though), which would have helped me

evaluate it.

Some parts of this paper were confusing, and I had to read some

sections several times to understand which aspect of the data were being

discussed (there are many different data sets, and sometimes it was

difficult to tell if they were talking about the average isotope values,

weekly values, precip values, groundwater values). I've tried to make

notes where I was most confused and would be helped with just a few extra

words explaining which data set is being discussed. One section in

particular is where Abbott talks about there being no relationship between

elevation and mean annual d18O in groundwater (pg. 8) and then later bases

his whole idea of being able to determine where the regions of year-round

recharge and wintertime recharge are on a linear relationship between

PRECIP d18O values and elevation--I think the distinction should have been

made more clear. Or maybe I was just not reading carefully enough.

I think this paper will be acceptable as soon as the confusing

sections are fixed and more of an effort made to explain why this study is

important. A more thorough explanation on how the boundary between

seasonal and year-round recharge was selected would help as well. On the

positive side, the graphs and figures are for the most part well-done and

help explain a lot of the text. Most of the writing is clear and concise,

and the conclusions are well-supported by the data (once those data are

fully understood).

Individual comments are forthcoming(I left them at home)

Review of "Stable isotopic measurements (H,O) constrain groundwater

recharge mechanisms in upland fractured bedrock aquifer, Vermont, USA" by

Michael Abbott et al.

The authors present a tight project that determines the groundwater

recharge zone of the Browns River, Vermont using del18 analysis of

precipitation and groundwater. They conclude that elevations above 800 m

are constant recharge zones while areas below 750 m are only seasonal

recharge zones, due mostly to larger vegetation depleting recent rainfall

from the vadose zone through transpiration. The authors also use some

assumptions to determine the percentage of yearly recharge from each zone.

The higher zone (36%) accounts for 58% of recharge while the lower

seasonal zone accounts for less than half of the recharge.

The data set is extensive and complete. Overall, the figures support the

text well. One exception is figure 8, that is not ref. in the text. I

strongly feel that figs 2, 4, and 5 should have better time control on the

x-axis. As they are now it is tough to separate out seasons and months

with just January labeled. Better care should be taken for refs. There

are refs. that are in the text, but not cited at the end, and there are

refs. cited at the end that are not in the text.

Overall I think this is an excellent study, and should be published.

There are only a few editorial suggestions listed below. I do feel

however, the authors should make it known that there assumptions on page

12 are not valid and should discuss how there calculations would change.

Also a different term than "continuous recharge" for upper elevations

would strengthen the paper.

Sorry that the numbers are not in sequential order but my thinking

capacity increased (I started to wake up) as I spent more time on this

paper, which caused me to backtrack at times.

1. I dont know what journal this is going to so many of my comments may be

a bit off base. But anyway, if this is an American journal do you think

you need the USA?

2. When I think of steep I think of a vertical distance over a horizontal

distance, not just a vertical distance. You could have 1030 m vertical

over 10,000 km horizontal and that is not very steep to me, but 1030 m

over 50 m horizontal is really steep.

3. Think you should give winter and summer averages?

4. Is this mixing from different rainfall events? If stated as so it

would make the reading a bit more clear.

5. Why is knowing the extent of urbanization important to this study? How

does the draw of wells impact your story?

6. Why?

7. Why did you take cumulative snow pack samples?

8. Need to include in ref. section.

9. Need to include in ref. section.

10. How valid are these assumptions, because we know that precip increases

with elevation and we also know that winter is drier than the summer

months in Vermont from history (your data in fig 2 shows this also). Cant

you also make a graph of precip vs. elevation since you have precip gauges

at different elevations up the mountain? And to go even further cant you

make a graph of season vs. precip also? How does this change your results

of your simple model?

11. Where does fig 8 belong?

12. Where does this figure come from?

13. When I think of Mansfield I think of seasonal recharge at high

elevations also. This is because it is cold and nasty for a good chuck of

December, January, and February. Couldnt recharge be seasonal for both

zones but just opposite each other? I dont think your data support this

though. Does that mean the recharge happens during the infrequent melts

at high elevation? But still recharge is shut off of a good chunk of time

during the winter, but has a few recharge periods. This parallels lower

vegetated areas that has recharge shut off during summer except for a few

large storms when recharge happens.

14. Could this also be from the trees taking up the precip. that may

effect the delta18 value?

15. Are these fluctuations from the infrequent melting events?

Abbott et al. use stable isotope ratios in groundwater to show the effects of evapotranspiration on bedrock aquifer recharge and flow patterns in a Vermont upland watershed. Over one thousand precipitation and groundwater samples were taken at varying elevations over a 1.5-year period. In colder months groundwater composition varies as much as 4.3 indicating rapid local recharge to the bedrock. In warmer months small variations in d180 readings at lower elevations is a result of the reduction of recharge caused by higher evapotranspiration rates. Continually throughout the year higher elevations in the watershed contribute to groundwater recharge due to less evapotranspiration as a result of colder temperatures and less vegetation.

The paper is very well written in terms of grammar, punctuation, and sentence structure. The data, interpretations, and figure captions are logically presented. The abstract does a good job of summarizing data and some key explanations/interpretations. I thought Figure 7, Burlington climate, soil moisture, and evapotranspiration was a particularly strong one. There are a lot of graphs attached, but since they are the data set I see no way to cut any from the paper.

I believe the paper should be excepted for publication in its current form. I had no major or minor suggestions for changing what is already written. However, I would have liked to have seen a short paragraph, in the Introduction, explaining what makes oxygen isotopes so useful for an experiment like this (i.e. why do the ratios change based on season and altitude). I am sure an expert in the field does not need this type of explanation, but it would help for someone with less background in this area.

Specific Comments:

1) I do not believe you ever refer to Figure 8 in the text.