Kindly refer :

VKSaxena NC Mondal and VSSingh, Identification of sea water ingress using Sr and B  in Krishna delta , India, Current Science Feb, 2004.

 Sample collection has no limit. it depend upon your requirement. Fix the sample collection points/stations and periodically monitor. Attempt to observe the variation in physico chemical parameters.

Geology maps and well data are critical, as well as pumping records that you can get. Groundwater that is unconfined may be different from confined aquifers, and each may have their own head (water table elevations) to consider.  Finding areas with deep cones of depression due to pumping in coastal areas are going to be important to consider.  Conversion of consumptive water uses from groundwater sources to surface water sources may be critical for some of these areas.  Those with confined aquifers with sufficient head differences should not have any immediate issues unless overused.  But a good geology and groundwater hydrology agency should be looking into these issues and insure uses are appropriate and not wasteful.  If you have some areas near the coast where the cone of depression is significant, installing a salinity sonde with water level, and other indicators such as made from Global water may be helpful in addressing salt water intrusion by placing a well in the cone of depression between the deepest point of depression and the coast.  If you visit many wells on a regular basis, and can afford only one sonde, take the sonde with you and transport it between stations, recording the conditions at each site.  The sonde will need to have a cable as deep as your deepest well and graduate marking on the cable for depth from the elevation of the well cap.  I don't know if Hobo company has any salinity data loggers, but if they do, they might be less expensive, but the Global Water sonde that I had was a quality instrument where you can set recording parameters.  Some of the large industrials should be willing to come up with some funds to help out as opposed to immediate restrictions until you obtain enough data to develop some answers.  Part of the problem is you may not know where the weakest points are that may be more susceptible to salt water intrusion.  Various coastal cities have done some salt water intrusion studies, so you might look into those, their geology types, methods, findings, etc.

Some links below:

http://www.ijesit.com/Volume%202/Issue%205/IJESIT201305_54.pdf

http://www.tandfonline.com/doi/pdf/10.1080/02626667909491835

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=8&cad=rja&uact=8&ved=0ahUKEwiT8IS6k6_MAhXKTSYKHTqIAoYQFghPMAc&url=http%3A%2F%2Fwww.journalijar.com%2Fuploads%2F732_IJAR-8686.pdf&usg=AFQjCNFdN5b2MoemnVd5Ggugf376IU-HfQ&bvm=bv.120551593,d.eWE

http://www.angelfire.com/nh/cpkumar/publication/gwman.pdf

http://swim-site.nl/pdf/swim20/file269-272.pdf

http://www.sciencedirect.com/science/article/pii/S2214241X15000073

http://www.bioone.org/doi/abs/10.2112/JCOASTRES-D-13-00021.1?journalCode=coas

http://www.diss.fu-berlin.de/docs/servlets/MCRFileNodeServlet/FUDOCS_derivate_000000005381/art_3A10.1007_2Fs12040-015-0600-y.pdf

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=4&cad=rja&uact=8&ved=0ahUKEwiT8IS6k6_MAhXKTSYKHTqIAoYQFggzMAM&url=http%3A%2F%2Fwww.editorialmanager.com%2Fjess%2Fdownload.aspx%3Fid%3D74237%26guid%3Da365b42b-2598-4332-b46b-21ce9c20cff1%26scheme%3D1&usg=AFQjCNEGP6CB-bgs6Jl5-DeiIt2McN_zdQ&bvm=bv.120551593,d.eWE

http://www.ga.gov.au/scientific-topics/water/groundwater/understanding-groundwater-resources/seawater-intrusion

https://www.researchgate.net/publication/237526761_MANAGEMENT_OF_GROUNDWATER_IN_SALT_WATER_INGRESS_COASTAL_AQUIFERS

http://www.ncbi.nlm.nih.gov/pubmed/21789532

http://olemiss.edu/sciencenet/saltnet/swica2/Oteri_ext.pdf

http://megphed.gov.in/knowledge/RainwaterHarvest/Chap11.pdf

Article Management of groundwater in salt water ingress coastal aquifers

Dear Prasoon, 

As previous colleagues mentioned, there are a lot of ratios used for salinity/seawater intrusion assessment. However, bear in mind that nearly all of them should be treated with caution when interpeted, due to interactions from other external sources. For example Na/Cl strongly depends on alternative Na sources, which can be abundant in natural environments. I believe that the most reliable of them is Cl/Br due to the conservative nature of the elements involved; however, even in this case you might have interferences (e.g. Br derived from pesticides or fire-returdants), so caution is always needed.

cheers 

 Depends on total dissolved solids with ionic concentration particularly, TDS versus Cl/HCO3 ratio is crucial factor for identifying seawater intrusion in the coastal aquifer. if values are grater than 1 indicate seawater intrusion

 Na/Cl, Cl/Br are also important ionic ratios for assessing  seawater intrusion.