Have you tried with Aspen Water? It can help you to select treatment methods according to water quality. Other tool are Water Pinch Analysis softwares find more here http://www.processint.com/download/software.
Dr Ramin Nabizadeh developed a new software for assessing water quality based on some water quality indices such as WQI, NSFWQI, and IWQS. The most important point is that it can be set for both ground water and surface water. Also it can be possible to define new index regarding to your aims or objects. Really user friendly and practical one.
I suggest using this software. Here I post the link of a paper about the software, so you can be connected to Dr Nabizadeh and ask for free trial source.
"Development of innovative computer software to facilitate the setup and computation of water quality index"
I would suggest you to use Aquachen 2010.1 software for water quality data interpretation and crack version of it also available so you can download for free. You just need to upload data file (excel file) irrespective of sample quantity and plot various plots viz., Piper, Durov, Ludwig, bivariate plot etc, besides correlation matrix assessment and water facies.
In the case of water quality index that can be developed yourself in excel sheet depending on you are purpose and data available, please go through attached paper for more details.
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