• USGS – Groundwater Software. Extremely useful: ModelMuse – GUI for MODFLOW, MT3D-MS and other.
• GSI Environmental – e.g. Excel tools for contamination hydrogeology.
• S.S. Papadopulos & Associates
• SERDP & ESTCP Simple tools for contamination hydrogeology.
• US EPA – tab "Models"
• Deltares – e.g. iMOD or geotechnical software. Almost all for free (e.g. FOSS).
• Hydrus-1D for unsaturated zone
• Geochemistry: PHREEQC, PhreePlot, ChemEQL, Thermoddem (Thermochemical and mineralogical tables for geochemical modeling)
• Else: Universitat Politècnica de Catalunya, Grupo de Hidrología Subterránea, waterlog.info, J.F.Devlin, Carlos Molano (MS Excels)

• Groundwater age mixtures and contaminant trends tool
• Calculators (GroundwaterSoftware.com)
• EPA On-line Tools for Site Assessment Calculation
• The fluid flow calculations (Ken Edwards, LMNO Engineering)
• pH calculator (aqion)
• Dissolved oxygen solubility tables (DOTABLES)
• Vapor intrusion screening levels calculator (VISL)
• An Earth scientist's periodic table of the elements and their ions (Source)

  Hydrograph Separation Tool (SepHydro)

• INOWAS DSS – Free web-based modeling platform for planning, management and optimisation of Managed Aquifer Recharge (MAR)
• ESlog – free online borehole profile
• Risk Assessment Information System

Microsoft Power Query for Excel: cross table (pivot table) to list (normalization) – details.

ASAP utilities

Molecular Weight Calculator

Example:

=MW("SO4")

Chemical formula can be in another cell (use reference without quotation marks):

=MW(A4)

Size matters: Co is not CO.