Peer review of: Groundwater level observations in 250,000 coastal US wells reveal scope of potential seawater in (2020)

Reviewers' comments: 

Reviewer #1 (Remarks to the Author): 

In their manuscript, “Groundwater level observations in 250,000 coastal USA wells reveal scope of 
potential seawater intrustion,” Jasechko et al. perform a massive analysis of water levels in wells near 
the coast and show that groundwater levels are below mean sea level in more than half of all wells 
along approximately 15% of the contiguous US coastline. The Atlantic and Gulf coasts are more 
“impacted” than the Pacific coast, likely because the Pacific coast has more relief and therefore more 
available drawdown before the water level drops below sea level. This analysis appears to be fairly 
robust, regardless of the range of well depths or distance from the coast used in the analysis, 
although there are lower water levels closer to the coast and in deeper wells. The extent of the 
problem was surprising to me, and therein lies the value of the manuscript. 

I have two major suggestions: 
The authors show in Figures S4 and S5 that the water level observations from drillers’ logs are rather 
coarse (the discrepancies with more accurate USGS and GAMA water levels are often on the order of a 
meter or so). The density of observations helps compensate for their relatively low accuracy. In 
contrast, the USGS and GAMA datasets are presumably much more accurate but far less dense. The 
methods of comparing the datasets were not as direct as I would have hoped, perhaps due to the low 
density of the USGS and GAMA datasets. I would encourage the authors to consider presenting some 
calculations, maps, or discussion of the number of USGS and GAMA wells that lie within 10 km of the 
coast, the number of 20-km coastal segments that they lie in, and the percentage (of the total) that 
have water levels below sea level. By comparison, how many other wells (from drilling logs) lie in the 
same 20-km coastal segments populated by a USGS or GAMA well, and what percentage of those 
wells have water levels below sea level? This would give readers a more direct understanding of why 
there is a need to use the less accurate drilling records to increase the volume of observations and 
how the results would change with the smaller, more accurate dataset. 

In Figure 2, the distance of 100 km from the coast is such a great length as to be potentially irrelevant 
to aquifer salinization unless saltwater first travels inland along surface water bodies (as is the case in 
parts of Florida). As a calculation, assuming a head difference of 10 m (one of the more extreme 
differences in Figure 2), a reasonably large hydraulic conductivity of 1 cm/s, and porosity of 0.3, it 
would take 950 years for salt to travel 100 km from the coast to a well. Upconing may provide a much 
shorter path to salinization, but it’s unclear where the freshwater-saltwater interface lies at a distance 
of 100 km inland or whether that deeper salt water is even sourced from modern seawater (as 
opposed to having recharged during previous glacial-interglacial cycles, etc). The authors could 
address this point by changing the distance from the coast that they analyze in Figure 2 (or any other 
places where 100 km is the distance of choice—I recognize that smaller distances are used throughout 
most of the manuscript, including key areas such as Figure 4 and Table 1). Regardless of distance 
used, it might be useful for the authors to provide some sort of salt travel timescale calculation like 
the one above to help frame the discussion of salinization more clearly. 

A few recommendations are below: 

L 121: Please provide range of years wells were drilled (2000-present) in this sentence. 
L 126: Please state that the dataset is NED here. 
L 129: If the water levels are lower than sea level, there is at least a local landward hydraulic gradient, 
but not necessarily a widespread regional one. Depending on recharge rate and hydraulic conductivity, 
there may still be a groundwater divide and seaward hydraulic gradient at the coast that together 
form a barrier to salinization. This point is alluded to in L 289-293 of the discussion, but it warrants 
earlier explanation. 
L 133: I would tend to call this concept “equivalent freshwater head” rather than the Ghyben-Herzberg 
principle but defer to the authors on terminology choice. Suggest quantifying the density effect on 
equivalent freshwater head. For a 30-m well filled with fresh water, the hydraulic gradient would still 
be landward unless the water level were greater than 0.6 m above sea level. 
Figure 2: Please add scale bars to the panels. 
Section S1: Why include inland states in data acquisition information? 

In reading the supplemental material, I applaud the authors on their thorough job bringing together 
large, publicly available datasets that are curated in different ways and have their own unique 
challenges. I can only imagine what a huge effort this was, and it has led to a very nice result that will 
be of broad interest to hydrogeologists, decision-makers, and coastal residents. 



Reviewer #2 (Remarks to the Author): 

This paper compiles and analyzes millions of groundwater level observations made since 2000 in 
coastal wells in USA. Through the simple analysis of the data, the authors found that the majority of 
observed groundwater levels are below sea level along more than 15% of the coastline in the US and 
concluded that landward hydraulic gradients characterize a substantial fraction of the US coastline. I 
applaud the authors on their efforts and time to collect such huge groundwater level data. These data, 
without doubt, indicate potentially that aquifers in some US coasts are under threats of seawater 
intrusion, and water resources managers should pay attention to the phenomena. 

Technically, the most concern I have is that the relationships among seawater intrusion, landward 
hydraulic gradients, and well groundwater level observations described in the paper. The observations 
were made within 10 km of the coast. Theoretically, the groundwater level below the sea level in an 
observation well located several kilometers from the coast does not necessarily cause a landward 
hydraulic gradient from the coastline, because there could be a hydraulic mound developed between 
observation wells and the sea due to factors such as topography, artificial recharge, and surface water 
bodies. Also, previous theoretical studies have demonstrated that if a well pumps groundwater with a 
safe rate, seawater would not intrude into the well (Strack, 1976). However, even the well pumps 
groundwater with a safe rate, the groundwater level in a neighboring observation well could be below 
the sea level because of the drawdown caused by the pumping well. 

In Supplementary Section S7, the authors classified the coastal aquifers as “submarine groundwater 
discharge” and “seawater intrusion or landward gradient”. It gives me the feeling that “Seawater 
intrusion” is equivalent to “landward gradient”, and “Seawater intrusion” and “submarine groundwater 
discharge” are exclusive. First, what is “seawater intrusion” defined here? Steady state or transient? 
The scales of seawater intrusion in different sites are different. How do you treat this issue? Do you 
define “seawater intrusion” for conditions under which only the interface moves inland? Second, note 
that seawater intrusion does not mean that a landward gradient occurs, which has been explained 
above. Third, under many conditions, seawater intrusion and submarine groundwater discharge are 
not exclusive and may occur simultaneously in the coastal subsurface. Figure S16 could give one an 
impression that the analysis based on the groundwater level data well matches largely the results of 
local-scale studies. However, I think the comparison is somewhat farfetched. 

Figure 3a shows that 44% of well water table observations made within 1km of the coast are below 
sea level. From my point of view, only the wells very close to the coast could show if there is a 
landward hydraulic gradient. The authors also need give the number of observations within 1 km of 
the coast, because they are crucial to conclude if there is a landward hydraulic gradient (or active 
seawater intrusion). 

The authors collected the groundwater level data since the year 2000, but did not show the trend of 
their variation in the recent 20 years, which I believe is important to water resources managers. 

In conclusion, the huge groundwater level data collected are valuable, which could potentially show 
the risk of extensive seawater intrusion and give a warning to water resources planners and managers 
in relevant coastal regions. However, using the water level observations made within 10 km of the 
coast to conjecture the gradient from the coast (or the seawater intrusion condition) is not very 
rigorous. 

Reference 

Strack, ODL (1976), Single-potential solution for regional interface problems in coastal aquifers, Water 
Resources Research, 12, 1165-1174. 




Reviewer #3 (Remarks to the Author): 

I'm attaching this review as a Word file. 

This timely study presents results from a national (US) analysis of groundwater levels in coastal basins, 
to estimate the extent of seawater intrusion that may be impacting associated aquifers and coastal 
communities and aquatic systems. This paper should be published. Please don't misinterpret my 
detailed questions and suggestions to indicate otherwise. I am interested in this topic, as are many 
others involved in fresh water research, engineering, and management. 

I have some suggestions for clarifying the analyses and language that I hope might provide more 
context and nuance. Some of these issues are addressed either in the main paper text or the 
supplement, but I'm suggesting that they be highlighted earlier in the study (in abbreviated form, can 
be explored in the supplement) to clarify both methods and interpretations of results. I consider these 
to be minor-to-modest revisions, not major, may not even need rereview. 

I am returning a marked-up PDF of the main paper, marking some of these issues and a few smaller 
suggestions. I am glad to be identified to the authors. 

Main concerns in presentation (readily addressable, not a reason to reject!): 

(1) As noted, there does not need to be a landward gradient in groundwater level for there to be 
seawater in an aquifer. But this is not made clear early in the paper. There are two distinct issues that 
are not commonly understood and should be noted earlier in the text. 
(a) Because of the difference in density, even under static conditions there is expected to be a 
seawater wedge that extends into coastal aquifers at depth (approximated crudely with the idealized 
Ghyben-Herzberg relation). Most coastal aquifers were not in static balance before people began to 
develop GW resources, there was flow to the ocean, and at dynamic steady state, this flow helped to 
shift the SW wedge seaward and/or change its shape. Still, the pre-development, initial condition in 
most basins should be a salt wedge of some kind at depth, despite the mean gradient being consistent 
with flow from land to the ocean. If you add tidal variations, this leads to mixing of FW and SW in the 
aquifer, as does convection at the salt front. So, coastal aquifers typically have some salty zones near 
the ocean. It is not clear if one considers this to be "intrusion," per se (definition in paper suggests 
that intrusion is active movement…but again, that can occur even dynamic steady state in terms of 
water levels), but it increases vulnerability because that salt is poised for landward advancement. 
(b) Even with groundwater levels above sea level, a lowering of groundwater level causes 
disproportionate landward movement of the salty wedge (=intrusion). Simple Ghyben-Herzberg 
analysis indicates 40:1 ratio of drop in water level to rise in the front of the wedge. Because most 
aquifers are more horizontal than vertical in dimension, a small rise in wedge interface usually results 
in a much larger lateral (landward) advance. 

For some reason, point (b) continues to elude a lot of engineers, scientists, and resource managers. I 
was at a public meeting about 10 years ago when a prominent engineer (who had received multiple 
awards for technical expertise), working as a consultant for an agency, asserted that there was no 
worry about intrusion in a coastal basin because water levels were above sea level. I could not believe 
he said this, and when I asked him about it (noting the points above), he dug in on it, would not 
acknowledge basic physics. 

This point is noted briefly in the text (line 62-63, and discussed later more extensively), but I think it 
needs to be clarified – I suggested a few simple edits in the manuscript text. Note that because of 
these issues, the current analysis is *conservative*. It highlights the most extreme cases, where there 
is GW level lower than SL. Intrusion could be more common. 

(2) Within coastal basins, active or accelerated seawater intrusion is a consequence of a mass 
imbalance. At least within CA, this imbalance is a consequence of three main factors: increased GW 
use (pumpage), shifting land use, changing climate. The first is emphasized in this paper, and 
certainly this is a common issue. But it is not quite right to talk about intrusion being "caused" by 
excess pumping. It is pumping in combination with the aggregate of inputs and outputs. 

Land use change towards urbanization tends to increase runoff and thus reduce infiltration and 
recharge (not a hard rule, but a tendency). So, you can hold pumping constant (or even reduce it) but 
have less recharge and that can lead to intrusion. Ag development in coastal basins can also lead to 
more runoff (although in the Central Valley, with water imports, there can be more recharge because 
of deep irrigation returns). Climate change has already resulted in an increase in rainfall intensity in 
CA (e.g., Russo et al., 2013 and other studies) – this is from the observational record from the last 
century, not just a simulation prediction. There can be same annual precip, but if more falls in smaller 
number of more intense storms, we get more runoff, less infiltration, less recharge. Once again, 
without having to change pumping, we can have GW budget imbalance. Whether the main issue in 
any particular basin (or part of a basin) is pumping or loss of recharge (from LU or CC) is spatially and 
temporarily variable – this needs to be assessed basin by basin. 

Russo, T. A., A. T. Fisher, and D. W. Winslow (2013), Regional and local increases in storm intensity in 
the San Francisco Bay Area, USA, between 1890 and 2010, J. Geophys. Res.-Atm., 18, 1-10, 
doi:10.1002/jgrd.50225. 



The following comments are listed in order of appearance 

Line 39. "… due to insufficient groundwater level measurements." 
This assertion is odd because the next sentence notes that the authors present results from analyzing 
250,000 water level measurements. I think that the lack of an earlier assessment is addressed by this 
study - certainly more data would be helpful, but if there were not enough data, then this study would 
not have been done. So, the issue really is that no one had previously done all the work required to 
assemble the data and analyze the results, not a lack of data, per se. 

Lines 58-61, especially "…here we focus on the latter.". 
See main points above. I think these lines are not quite right. The observational data and main 
analyses are a series of synoptic views of GW levels in coastal basins. Full stop. The link from there to 
changes in pumpage, climate, and land use are not really addressed in this study, and probably this 
would require basin/sub-basin scale assessment, and detailed analysis of pumping patterns (in time 
and space), climate, land use, etc. 

Lines 70-110. I would present much of this information after the main analyses are presented, and 
instead, bring up front a more nuanced explanation of SW intrusion, heterogeneity, and fresh-water 
heads (e.g., Lines 280-281, 295-301, among others). 

Line 98: This line makes a case for the three-dimensional nature of intrusion, but the analysis is really 
1D or 2D, especially because of heterogeneity in properties and the distribution of pumping. The issue 
of heterogeneity is highlighted in Supplement p. 55, for Monterey Bay Region, but I don’t think we 
know how common spatial variability is in intrusion patterns more broadly – my guess is that it is 
common, because aquifer properties, recharge, and pumping are heterogeneously distributed. 

In my region, where SW intrusion is common, the same aquifer system that is intruded in one location 
is not intruded just a few km away. I've seen this in GW quality records, big differences in chemistry 
at the same time – one area intruded, nearby area looks fine. Of course, at the large scale, the aquifer 
is intruded (no question), but I have been faced with upset well owners who argue that the intrusion is 
not impacting their wells, so they should not be held accountable (financially) for intrusion elsewhere. 
They have a point. In one case, it appears that un-intruded areas may have wells screened within a 
paleo channel that is directly connected to a recharge area. 

None of this contradicts the key points being made in this paper, but I think some of this nuance is 
important because aquifers are not continuous, flat layers with uniform properties – Graham Fogg 
likes to say, "Most of the aquifer is not an aquifer," referring to less conductive regions even within 
highly transmissive aquifers. 

Lines 111-125. It is not clear how biases in where and when data was collected from wells may impact 
this study. For coastal basins that are worried about SW intrusion, they often emphasize monitoring of 
wells close to the coast. On the one hand, this could lead to a bias in elevated TDS levels, but water 
levels tend to be "pinned" by sea level very close to the ocean, so somewhat more inland wells might 
be more indicative of the lowering of GW levels over time. Also, those coastal networks are likely 
monitored more frequently than are inland wells. And even if a production well is shut down for 
measurement, then one nearby may not be. 

Lines 193-194, text says, "More than half of all recorded well water levels that are below sea level are 
found within 8 km of the coast." Is this more than half of well readings or more than half of wells from 
which readings have been taken?" I think it might be the latter, as Line 199 says, "We report our 
results as percentages of wells exhibiting landward gradients…" But if this is the case, then language 
throughout should be checked and clarified. If it is more than half the readings, then there could be a 
bias if coastal wells are checked more often. 
Other examples that are confusing include Lines 215-224 – this section seems to emphasize 
percentages of water well level observations, rather than wells, from which observations have been 
made. This pops up again on 227 and 238. Table 1 suggests that these might be percentages of wells, 
not level measurements. Will be good to clarify. 

Not sure if this is possible, but perhaps for selected basins or regions, authors could indicate what 
fraction of aquifer area is impacted. Maybe that will reduce bias from clustering of wells (in space) 
and/or inconsistent time of measurements/sampling. 

Lines 126-138. As noted in S5, many well records in CA do not include well locations beyond TRS 
designation (putting wells in the center of a designated Section). This can lead to big errors in 
estimates of well or screen depth when comparing well "locations" to a highly variable DEM. I'm not 
sure if/how how this applies in other regions – are well locations in other regions properly 
georeferenced? 

130-134. These are equivalent freshwater gradients, based on equivalent fresh water heads, need to 
clarify. Point is mentioned on Lines 302-309 

Figure 1 caption – seems like too much discussion, better to simplify the caption and refer readers 
seeking more information to text/supplement sections. 

280-281, 295-301 Need to emphasize these considerations earlier in the paper. In many ways, this 
analysis is conservative in terms of identifying problems. 

Line 364. "Abstracting" is an odd usage. How about "extracting" instead? 

- What I find especially surprising is that only 15% of the coastline seems to be impacted, particularly 
when you consider that wells are most common where there is the most development, and monitoring 
is likely to be most common where there are problems. I would have guessed that 30 or 40 or 50% of 
coastal basins have problems. 

- A final comment on the overall results. It seems like the basis for problems may be different in the 
three main regions: west, south/southeast, northeast, with coastal aquifers from the latter two areas 
being more impacted. 
- The west is notable for small, sedimentary coastal basins and steep topography and ongoing uplift, 
with a narrow continental shelf and near-shore oceanic depths. This means that aquifers are exposed 
to the ocean to considerable depth close to land, but aquifers are highly layered, and shallow aquifers 
tend to be distinct from deeper ones. 
- The south/southeast is notable for wide coastal plains, with a broad continental shelf, could be 
uplifting or (more likely) sinking due to glacial rebound (uplift to north, sinking to south). 
- The northeast is like the south/southeast, but the aquifers there were also subject to recharge at the 
base of continental ice sheets during the LGM, so those aquifers were somewhat "charged" with fresh 
water under the shelf (e.g., Person et al., 2003). Now these aquifers are not receiving as much 
recharge from inland as they were when there were ice sheets. 

Person, M., B. Dugan, J. B. Swenson, L. Urbano, C. Sttot, J. Taylor, and M. Willett (2003), Pleistocene 
hydrogeology of the Atlantic continental shelf, New England, Geol. Soc. Am. Bull., 115, 1324–1343. 

I wonder how much of the difference between current conditions (more of the south/southeast and 
northeast indicating conditions favoring intrusion) results from these physiographic and boundary 
conditions. Of course, it is more complicated than a single set of conditions: development, climate and 
other factors must also play roles. But all that said, a broad, thick, flat aquifer seems like it would be 
more susceptible to intrusion, all else equal…