DataTable ModelData = getModelData() // Returns all records to search.
...
EnumerableRowCollection<DataRow> modelRows = (from model in ModelData.AsEnumerable()
                                              where (model.Field<object>(GeographicalKey) ?? (object)String.Empty).ToString() == GeographicCode
                                              select model);
ModelResults = modelRows.Any() ? modelRows.CopyToDataTable() : ModelData.Clone();

for (int i = 0; i < ModelResults.Rows.Count; i++)
{
    for (int j = 0; j < ModelData.Rows.Count - 1; j++)
    {
        if (ModelResults.Rows[i]["Request ID"].ToString() == ModelData.Rows[j]["Request ID"].ToString())
        {
            ModelData.Rows[j]["Collected"] = "1";
        }
    }
}

var collectedRequestIds = new HashSet<string>(
    modelResults.AsEnumerable().Select(row => row["Request ID"].ToString()));

modelData.AsEnumerable()
    .Where(collectedRequestIds.Contains(row => row["Request ID"].ToString())))
    .ToList().ForEach(row => row["Collected"] = "1");