healthMonitoring Assembly load error

I am trying to use healthMonitoring. I am getting this error at start up:  Could not load file or assembly 'System.Web' or one of its dependencies.
The error is occurring at this line of the web.config load:
Line 162:              <remove name="SqlWebEventProvider"></remove>
Line 163:              <add name="SqlWebEventProvider"
Line 164:                    type="System.Web.Management.SqlWebEventProvider,System.Web"
Line 165:                    connectionStringName="ApplicationServices"
Line 166:             ...