Group By in DataTable in C# Linq in SharePoint 2010

//Get data from SharePoint
            SPWeb _web = GetWeb();
            SPList list = _web.Lists["List"];
            SPQuery query = new SPQuery();
            query.Query = "" + _user.Name.Trim()
                          + ""
+ selectedYear.Trim() + "";
DataTable dt = list.GetItems(query).GetDataTable();

//Get DataRows
            if (dt != null && dt.Rows.Count > 0)
            {
                //Group the data
                var groupedRows = from row in dt.AsEnumerable()
                                  group row by row.Field(dt.Columns["Department"]) into grp
                                  orderby grp.Key
                                  select new
                                  {
                                      Name = grp.Key,
                                      _grp = grp
                                  };
                foreach (var grp in groupedRows)
                {
                    AccordionPane currentPane = CreateAccordinHeaderPane(mainacc, grp.Name);
                    groupedRows.ToList();
                    List rows = grp._grp.ToList();
                    AddContentToPane(currentPane, rows);
                }
            }
DataTable dt = new DataTable();
            dt = rows.CopyToDataTable(); -> Gets