Journal of Hydrology: Regional Studies (2017) 39–40 Contents lists available at ScienceDirect Journal of Hydrology: Regional Studies journal homepage: www.elsevier.com/locate/ejrh Peer Review Report Peer review report on “Groundwater-derived Nutrient and Trace Element Transport to a Nearshore Kona Coral Ecosystem: Experimental Mixing Model Results” Original Submission Recommendation Major Revision Comments to Author Review of MS #EJRH-D-15-00130, “Groundwater-derived Nutrient and Trace Element Transport to a Nearshore Kona Coral Ecosystem: Experimental Mixing Model Results” by Prouty et al This article addresses the issue of nutrient and dissolved metal transport into Kona coastal waters from submarine groundwater discharge (SGD) The authors take an interesting approach to investigating mixing within the subterranean estuary: they collect samples of two groundwater endmembers and mix different aliquots of these endmembers with local seawater to investigate whether or not mixing is conservative Their second main focus is on the role of colloids, defined as suspended particulate matter with a diameter of between 0.02 and 0.45 microns, in the delivery of four metals (Mo, V, U and Ba) to coastal waters via SGD The questions that the authors investigate are scientifically interesting, and moreover have important management implications on the rapidly developing Kona Coast, and, in general, the article is well written and the figures and tables are professional looking and easy to understand However, in my opinion the manuscript has some serious shortcomings that need to be addressed before it can be published In addition, I have noted some more minor revisions that should be made Major issues 1) The authors appear to be modeling mixing within the subterranean estuary by mixing different aliquots of two groundwater endmembers with local seawater in large bottles for two hours This is an interesting approach that I have never seen used before, and it may yield valuable insight about mixing in the subterranean estuary However, it is at best a major simplification of what goes on in the actual subterranean estuary Some of the main features of the subterranean estuary that are not captured by this approach are: the physicochemical properties of the aquifer substrate (mainly porous and/or fractured basalt flows), redox conditions within the aquifer, temperature and temperature gradients within the aquifer, and the interplay of tidal mixing and seaward groundwater flow driven by the hydraulic gradient These simplifying assumptions must be clearly stated and the limitations they imply for how the results should be interpreted and applied must be clearly and thoroughly discussed Also, there needs to be some discussion of how this approach compares to detailed field experiments when it comes to understanding nonconservative mixing in subterranean estuaries and coastal waters Or is detailed sampling just not possible here because of the hard rock aquifer .? 2) The authors sampled a transect in Honokohau Harbor with a high degree of spatial resolution (both in terms of distance from the head of the harbor, and vertical profiles) However, they either did not collect nutrient and dissolved metal data from these sampling points, or they choose not to present the data here This seems really strange to me It would be DOI of the original article:http://dx.doi.org/10.1016/j.ejrh.2015.12.058 2214-5818/$ – see front matter http://dx.doi.org/10.1016/j.ejrh.2016.11.028 40 3) 4) 5) 6) Peer Review Report / Journal of Hydrology: Regional Studies (2017) 39–40 helpful to be able to compare the variation in nutrient and metal concentrations along real salinity gradients in the harbors to those you observed in your lab mixing experiments If the patterns are similar (especially with the HH2 monitor well mixing experiment), it would support the validity of your approach If they are different, then you would need to explore what is behind that difference The acidification step in the phosphate mixing experiment is not explained sufficiently Presumably the acidification is supposed to represent a process or condition that could occur either in the subterranean estuary or after groundwater discharges into the coastal ocean, but I read the article twice and did not get a sense of what that process or condition is supposed to be The acidification step is not mentioned at all in the methods, and important information, such as what pH the samples were acidified to and what acid was used to accomplish this, are left out There is no explanation of why the particular metals Ba, Mo, U and V were measured in this experiment This needs to be clearly explained The non-conservative mixing of phosphate in the mixing experiments is a dramatic and interesting result However, the explanation for it is not completely clear It seems like the authors are saying that a change in surface complexation explains the non-conservative release of phosphate at intermediate salinities But what surfaces are these? Surfaces of suspended particulate matter/colloids? Of the bottles in the experiment? Sediment or rock surfaces within the aquifer? I also think the introduction is too general and needs to contain more specific information relevant to the experiment that was conducted Make it really clear what parts of this study/experiment build on the results of prior work (e.g the fairly abundant literature about how SGD supplies large nutrient subsidies to coastal waters on the Kona Coast) and which parts are new Have others investigated the role of colloids in metal/nutrient loading by SGD before? Minor issues 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20) 21) ă Lines 66-71, With high permeability .Kaloko Honokohau NHP.This is a poorly structured, run-on sentence Please revise Line 115: Put a comma between “downward” and “contributing” Lines 118-119: “freshwater lens” does not need to be hyphenated Line 165: Why was the minimum hours? Line 182: Change “Total dissolved nitrogen (TDN) by measured by” to “Total dissolved nitrogen (TDN) was measured by” Lines 200-201: “The relation between salinity and temperature (Fig 2a) is semi-conservative” I don’t know if it makes sense to talk about temperature being conservative Maybe it would be better to say that both increase in a steady and generally linear fashion with distance from the harbor terminus, or something similar Lines 223-224: “Silicic acid concentrations dominated the total nutrient input” I don’t think it makes sense to talk about the total nutrient input, because the sum is not really relevant, and phytoplankton/algae have different requirements for each nutrient Line 229: “inorganic N” does not need a hyphen Line 237: Change “greater than HH monitor well no endmember” to “greater than the HH monitor well no endmember” Lines 243-244: Change this sentence so that it is clear you are talking about the results of previous work (which it seems like you are, based on the citations) rather than the results of your study Lines 252-253: Change “can be focused as point-source discharges rather than diffuse groundwater flow” to “can occur as point-source discharges in addition to more diffuse groundwater flow” Lines 251-255: I think that this statement would be just as true for diffuse groundwater seepage as for distinct groundwater springs Lines 256-257: Change “are a reflection” to “are likely a reflection” Lines 259-261: This information should be in the introduction Change lines 271-272 to: “Although the WWTF appeared to be a point source of nutrients, that was not true of trace elements” or something similar Lines 306-307: “have been demonstrated in both lab and field studies” Please cite lab and field studies (not just one study) Lines 321-322: “The effect of pH on adsorption of phosphate has been previously observed in seawater for goethite (FeO(OH))ă Explain this more Is the goethite in sediments or aquifer substrate, or is it dissolved in the water? Lines 334-335: “This pattern was explained by a rapid drop in SPM at a salinity of 15” Is this in the lab experiment, or in the harbor transect? What is the mechanism for the drop in SPM? Lines 336-339: I don’t really understand what you are saying here Was SPM present? You can’t invoke it as an explanation if it wasn’t there Fig seems to replicate information that is already in Table 4, and I think it should be removed Figures and and the bottom half of Fig seem to contain most of the same information Can you merge them into one figure explaining non-conservative mixing of phosphate? Anonymous ... 4) 5) 6) Peer Review Report / Journal of Hydrology: Regional Studies (20 17) 39–40 helpful to be able to compare the variation in nutrient and metal concentrations along real salinity gradients... what that process or condition is supposed to be The acidification step is not mentioned at all in the methods, and important information, such as what pH the samples were acidified to and what acid... I don’t really understand what you are saying here Was SPM present? You can’t invoke it as an explanation if it wasn’t there Fig seems to replicate information that is already in Table 4, and