How should I prepare data for semiovariogram analysis in ArcGIS with large sample sizes?

I am working on a statewide project to create surface water quality grids from point samples. In some instances, we have large sample sizes (n>50,000) which is problematic when creating semiovariograms in ArcGIS' Geostatistical Analyst. See my sample locations for groundwater pH. I'm looking for some literature on how to sub-sample my data prior to interpolation so I can look at the semiovariogram, trends and geostatistics.